diff --git a/CHANGELOG.md b/CHANGELOG.md
index 09a06fe9..a9b09c11 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 * `__radd__` and other `__r*__` methods now correctly work with operators. [#839](https://github.com/PyO3/pyo3/pull/839)
+* Garbage Collector causing random panics when traversing objects that were mutably borrowed. [#855](https://github.com/PyO3/pyo3/pull/855)
 
 ## [0.9.2]
 
@@ -42,6 +43,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 * `PyAny` is now on the top level module and prelude. [#816](https://github.com/PyO3/pyo3/pull/816)
 
 ### Added
+
 * `PyCell`, which has RefCell-like features. [#770](https://github.com/PyO3/pyo3/pull/770)
 * `PyClass`, `PyLayout`, `PyClassInitializer`. [#683](https://github.com/PyO3/pyo3/pull/683)
 * Implemented `IntoIterator` for `PySet` and `PyFrozenSet`. [#716](https://github.com/PyO3/pyo3/pull/716)
@@ -113,14 +115,12 @@ and `PyString::to_string_lossy` [#642](https://github.com/PyO3/pyo3/pull/642).
 * Remove `__contains__` and `__iter__` from PyMappingProtocol. [#644](https://github.com/PyO3/pyo3/pull/644)
 * Fix proc-macro definition of PySetAttrProtocol. [#645](https://github.com/PyO3/pyo3/pull/645)
 
-
 ## [0.8.1]
 
 ### Added
 
  * Conversion between [num-bigint](https://github.com/rust-num/num-bigint) and Python int. [#608](https://github.com/PyO3/pyo3/pull/608)
 
-
 ### Fixed
 
  * Make sure the right Python interpreter is used in OSX builds. [#604](https://github.com/PyO3/pyo3/pull/604)
@@ -403,7 +403,6 @@ Yanked
 
 * Remove use of now unneeded 'AsciiExt' trait
 
-
 ## [0.2.2] - 09-26-2017
 
 ### Changed
diff --git a/README.md b/README.md
index 7d715742..dda19fd2 100644
--- a/README.md
+++ b/README.md
@@ -128,13 +128,18 @@ fn main_(py: Python) -> PyResult<()> {
 Our guide has [a section](https://pyo3.rs/master/python_from_rust.html) with lots of examples
 about this topic.
 
-## Examples and tooling
+## Tools and libraries
+ * [maturin](https://github.com/PyO3/maturin) _Zero configuration build tool for Rust-made Python extensions_.
+ * [setuptools-rust](https://github.com/PyO3/setuptools-rust) _Setuptools plugin for Rust support_.
+ * [pyo3-built](https://github.com/PyO3/pyo3-built) _Simple macro to expose metadata obtained with the [`built`](https://crates.io/crates/built) crate as a [`PyDict`](https://pyo3.github.io/pyo3/pyo3/struct.PyDict.html)_
+ * [rust-numpy](https://github.com/PyO3/rust-numpy) _Rust binding of NumPy C-API_
+ * [dict-derive](https://github.com/gperinazzo/dict-derive) _Derive FromPyObject to automatically transform Python dicts into Rust structs_
+
+## Examples
 
  * [examples/word-count](examples/word-count) _Counting the occurrences of a word in a text file_
  * [hyperjson](https://github.com/mre/hyperjson) _A hyper-fast Python module for reading/writing JSON data using Rust's serde-json_
- * [rust-numpy](https://github.com/PyO3/rust-numpy) _Rust binding of NumPy C-API_
  * [html-py-ever](https://github.com/PyO3/setuptools-rust/tree/master/html-py-ever) _Using [html5ever](https://github.com/servo/html5ever) through [kuchiki](https://github.com/kuchiki-rs/kuchiki) to speed up html parsing and css-selecting._
- * [pyo3-built](https://github.com/PyO3/pyo3-built) _Simple macro to expose metadata obtained with the [`built`](https://crates.io/crates/built) crate as a [`PyDict`](https://pyo3.github.io/pyo3/pyo3/struct.PyDict.html)_
  * [point-process](https://github.com/ManifoldFR/point-process-rust/tree/master/pylib) _High level API for pointprocesses as a Python library_
  * [autopy](https://github.com/autopilot-rs/autopy) _A simple, cross-platform GUI automation library for Python and Rust._
    * Contains an example of building wheels on TravisCI and appveyor using [cibuildwheel](https://github.com/joerick/cibuildwheel)
@@ -144,7 +149,6 @@ about this topic.
    * Contains an example of building wheels on Azure Pipelines
  * [fastuuid](https://github.com/thedrow/fastuuid/) _Python bindings to Rust's UUID library_
  * [python-ext-wasm](https://github.com/wasmerio/python-ext-wasm) _Python library to run WebAssembly binaries_
- * [dict-derive](https://github.com/gperinazzo/dict-derive) _Derive FromPyObject to automatically transform Python dicts into Rust structs_
  * [mocpy](https://github.com/cds-astro/mocpy) _Astronomical Python library offering data structures for describing any arbitrary coverage regions on the unit sphere_
  * [tokenizers](https://github.com/huggingface/tokenizers/tree/master/bindings/python) _Python bindings to the Hugging Face tokenizers (NLP) written in Rust_
 
diff --git a/guide/src/get_started.md b/guide/src/get_started.md
index 4c55f896..2a17b85c 100644
--- a/guide/src/get_started.md
+++ b/guide/src/get_started.md
@@ -123,13 +123,18 @@ fn main_(py: Python) -> PyResult<()> {
 Our guide has [a section](https://pyo3.rs/master/python_from_rust.html) with lots of examples
 about this topic.
 
-## Examples and tooling
-
- * [examples/word-count](https://github.com/PyO3/pyo3/tree/master/examples/word-count) _Counting the occurrences of a word in a text file_
- * [hyperjson](https://github.com/mre/hyperjson) _A hyper-fast Python module for reading/writing JSON data using Rust's serde-json_
- * [rust-numpy](https://github.com/PyO3/rust-numpy) _Rust binding of NumPy C-API_
- * [html-py-ever](https://github.com/PyO3/setuptools-rust/tree/master/html-py-ever) _Using [html5ever](https://github.com/servo/html5ever) through [kuchiki](https://github.com/kuchiki-rs/kuchiki) to speed up html parsing and css-selecting._
+## Tools and libraries
+ * [maturin](https://github.com/PyO3/maturin) _Zero configuration build tool for Rust-made Python extensions_.
+ * [setuptools-rust](https://github.com/PyO3/setuptools-rust) _Setuptools plugin for Rust support_.
  * [pyo3-built](https://github.com/PyO3/pyo3-built) _Simple macro to expose metadata obtained with the [`built`](https://crates.io/crates/built) crate as a [`PyDict`](https://pyo3.github.io/pyo3/pyo3/struct.PyDict.html)_
+ * [rust-numpy](https://github.com/PyO3/rust-numpy) _Rust binding of NumPy C-API_
+ * [dict-derive](https://github.com/gperinazzo/dict-derive) _Derive FromPyObject to automatically transform Python dicts into Rust structs_
+
+## Examples
+
+ * [examples/word-count](examples/word-count) _Counting the occurrences of a word in a text file_
+ * [hyperjson](https://github.com/mre/hyperjson) _A hyper-fast Python module for reading/writing JSON data using Rust's serde-json_
+ * [html-py-ever](https://github.com/PyO3/setuptools-rust/tree/master/html-py-ever) _Using [html5ever](https://github.com/servo/html5ever) through [kuchiki](https://github.com/kuchiki-rs/kuchiki) to speed up html parsing and css-selecting._
  * [point-process](https://github.com/ManifoldFR/point-process-rust/tree/master/pylib) _High level API for pointprocesses as a Python library_
  * [autopy](https://github.com/autopilot-rs/autopy) _A simple, cross-platform GUI automation library for Python and Rust._
    * Contains an example of building wheels on TravisCI and appveyor using [cibuildwheel](https://github.com/joerick/cibuildwheel)
@@ -139,4 +144,5 @@ about this topic.
    * Contains an example of building wheels on Azure Pipelines
  * [fastuuid](https://github.com/thedrow/fastuuid/) _Python bindings to Rust's UUID library_
  * [python-ext-wasm](https://github.com/wasmerio/python-ext-wasm) _Python library to run WebAssembly binaries_
- * [dict-derive](https://github.com/gperinazzo/dict-derive) _Derive FromPyObject to automatically transform Python dicts into Rust structs_
+ * [mocpy](https://github.com/cds-astro/mocpy) _Astronomical Python library offering data structures for describing any arbitrary coverage regions on the unit sphere_
+ * [tokenizers](https://github.com/huggingface/tokenizers/tree/master/bindings/python) _Python bindings to the Hugging Face tokenizers (NLP) written in Rust_
diff --git a/pyo3-derive-backend/src/defs.rs b/pyo3-derive-backend/src/defs.rs
index 145b56f5..02a5ca74 100644
--- a/pyo3-derive-backend/src/defs.rs
+++ b/pyo3-derive-backend/src/defs.rs
@@ -7,9 +7,44 @@ pub struct Proto {
     pub py_methods: &'static [PyMethod],
 }
 
+impl Proto {
+    pub(crate) fn get_proto<Q>(&self, query: Q) -> Option<&'static MethodProto>
+    where
+        Q: PartialEq<&'static str>,
+    {
+        self.methods.iter().find(|m| query == m.name())
+    }
+    pub(crate) fn get_method<Q>(&self, query: Q) -> Option<&'static PyMethod>
+    where
+        Q: PartialEq<&'static str>,
+    {
+        self.py_methods.iter().find(|m| query == m.name)
+    }
+}
+
+// TODO(kngwyu): Currently only __radd__-like methods use METH_COEXIST to prevent
+// __add__-like methods from overriding them.
 pub struct PyMethod {
     pub name: &'static str,
     pub proto: &'static str,
+    pub can_coexist: bool,
+}
+
+impl PyMethod {
+    const fn coexist(name: &'static str, proto: &'static str) -> Self {
+        PyMethod {
+            name,
+            proto,
+            can_coexist: true,
+        }
+    }
+    const fn new(name: &'static str, proto: &'static str) -> Self {
+        PyMethod {
+            name,
+            proto,
+            can_coexist: false,
+        }
+    }
 }
 
 pub const OBJECT: Proto = Proto {
@@ -73,18 +108,9 @@ pub const OBJECT: Proto = Proto {
         },
     ],
     py_methods: &[
-        PyMethod {
-            name: "__format__",
-            proto: "pyo3::class::basic::FormatProtocolImpl",
-        },
-        PyMethod {
-            name: "__bytes__",
-            proto: "pyo3::class::basic::BytesProtocolImpl",
-        },
-        PyMethod {
-            name: "__unicode__",
-            proto: "pyo3::class::basic::UnicodeProtocolImpl",
-        },
+        PyMethod::new("__format__", "pyo3::class::basic::FormatProtocolImpl"),
+        PyMethod::new("__bytes__", "pyo3::class::basic::BytesProtocolImpl"),
+        PyMethod::new("__unicode__", "pyo3::class::basic::UnicodeProtocolImpl"),
     ],
 };
 
@@ -120,14 +146,14 @@ pub const ASYNC: Proto = Proto {
         },
     ],
     py_methods: &[
-        PyMethod {
-            name: "__aenter__",
-            proto: "pyo3::class::pyasync::PyAsyncAenterProtocolImpl",
-        },
-        PyMethod {
-            name: "__aexit__",
-            proto: "pyo3::class::pyasync::PyAsyncAexitProtocolImpl",
-        },
+        PyMethod::new(
+            "__aenter__",
+            "pyo3::class::pyasync::PyAsyncAenterProtocolImpl",
+        ),
+        PyMethod::new(
+            "__aexit__",
+            "pyo3::class::pyasync::PyAsyncAexitProtocolImpl",
+        ),
     ],
 };
 
@@ -165,14 +191,14 @@ pub const CONTEXT: Proto = Proto {
         },
     ],
     py_methods: &[
-        PyMethod {
-            name: "__enter__",
-            proto: "pyo3::class::context::PyContextEnterProtocolImpl",
-        },
-        PyMethod {
-            name: "__exit__",
-            proto: "pyo3::class::context::PyContextExitProtocolImpl",
-        },
+        PyMethod::new(
+            "__enter__",
+            "pyo3::class::context::PyContextEnterProtocolImpl",
+        ),
+        PyMethod::new(
+            "__exit__",
+            "pyo3::class::context::PyContextExitProtocolImpl",
+        ),
     ],
 };
 
@@ -222,14 +248,11 @@ pub const DESCR: Proto = Proto {
         },
     ],
     py_methods: &[
-        PyMethod {
-            name: "__del__",
-            proto: "pyo3::class::context::PyDescrDelProtocolImpl",
-        },
-        PyMethod {
-            name: "__set_name__",
-            proto: "pyo3::class::context::PyDescrNameProtocolImpl",
-        },
+        PyMethod::new("__del__", "pyo3::class::context::PyDescrDelProtocolImpl"),
+        PyMethod::new(
+            "__set_name__",
+            "pyo3::class::context::PyDescrNameProtocolImpl",
+        ),
     ],
 };
 
@@ -283,10 +306,10 @@ pub const MAPPING: Proto = Proto {
             proto: "pyo3::class::mapping::PyMappingReversedProtocol",
         },
     ],
-    py_methods: &[PyMethod {
-        name: "__reversed__",
-        proto: "pyo3::class::mapping::PyMappingReversedProtocolImpl",
-    }],
+    py_methods: &[PyMethod::new(
+        "__reversed__",
+        "pyo3::class::mapping::PyMappingReversedProtocolImpl",
+    )],
 };
 
 pub const SEQ: Proto = Proto {
@@ -579,10 +602,9 @@ pub const NUM: Proto = Proto {
             pyres: false,
             proto: "pyo3::class::number::PyNumberIModProtocol",
         },
-        MethodProto::Ternary {
+        MethodProto::Binary {
             name: "__ipow__",
-            arg1: "Other",
-            arg2: "Modulo",
+            arg: "Other",
             pyres: false,
             proto: "pyo3::class::number::PyNumberIPowProtocol",
         },
@@ -651,81 +673,58 @@ pub const NUM: Proto = Proto {
             pyres: true,
             proto: "pyo3::class::number::PyNumberFloatProtocol",
         },
-        MethodProto::Unary {
-            name: "__round__",
-            pyres: true,
-            proto: "pyo3::class::number::PyNumberRoundProtocol",
-        },
         MethodProto::Unary {
             name: "__index__",
             pyres: true,
             proto: "pyo3::class::number::PyNumberIndexProtocol",
         },
+        MethodProto::Binary {
+            name: "__round__",
+            arg: "NDigits",
+            pyres: true,
+            proto: "pyo3::class::number::PyNumberRoundProtocol",
+        },
     ],
     py_methods: &[
-        PyMethod {
-            name: "__radd__",
-            proto: "pyo3::class::number::PyNumberRAddProtocolImpl",
-        },
-        PyMethod {
-            name: "__rsub__",
-            proto: "pyo3::class::number::PyNumberRSubProtocolImpl",
-        },
-        PyMethod {
-            name: "__rmul__",
-            proto: "pyo3::class::number::PyNumberRMulProtocolImpl",
-        },
-        PyMethod {
-            name: "__rmatmul__",
-            proto: "pyo3::class::number::PyNumberRMatmulProtocolImpl",
-        },
-        PyMethod {
-            name: "__rtruediv__",
-            proto: "pyo3::class::number::PyNumberRTruedivProtocolImpl",
-        },
-        PyMethod {
-            name: "__rfloordiv__",
-            proto: "pyo3::class::number::PyNumberRFloordivProtocolImpl",
-        },
-        PyMethod {
-            name: "__rmod__",
-            proto: "pyo3::class::number::PyNumberRModProtocolImpl",
-        },
-        PyMethod {
-            name: "__rdivmod__",
-            proto: "pyo3::class::number::PyNumberRDivmodProtocolImpl",
-        },
-        PyMethod {
-            name: "__rpow__",
-            proto: "pyo3::class::number::PyNumberRPowProtocolImpl",
-        },
-        PyMethod {
-            name: "__rlshift__",
-            proto: "pyo3::class::number::PyNumberRLShiftProtocolImpl",
-        },
-        PyMethod {
-            name: "__rrshift__",
-            proto: "pyo3::class::number::PyNumberRRShiftProtocolImpl",
-        },
-        PyMethod {
-            name: "__rand__",
-            proto: "pyo3::class::number::PyNumberRAndProtocolImpl",
-        },
-        PyMethod {
-            name: "__rxor__",
-            proto: "pyo3::class::number::PyNumberRXorProtocolImpl",
-        },
-        PyMethod {
-            name: "__ror__",
-            proto: "pyo3::class::number::PyNumberROrProtocolImpl",
-        },
-        PyMethod {
-            name: "__complex__",
-            proto: "pyo3::class::number::PyNumberComplexProtocolImpl",
-        },
-        PyMethod {
-            name: "__round__",
-            proto: "pyo3::class::number::PyNumberRoundProtocolImpl",
-        },
+        PyMethod::coexist("__radd__", "pyo3::class::number::PyNumberRAddProtocolImpl"),
+        PyMethod::coexist("__rsub__", "pyo3::class::number::PyNumberRSubProtocolImpl"),
+        PyMethod::coexist("__rmul__", "pyo3::class::number::PyNumberRMulProtocolImpl"),
+        PyMethod::coexist(
+            "__rmatmul__",
+            "pyo3::class::number::PyNumberRMatmulProtocolImpl",
+        ),
+        PyMethod::coexist(
+            "__rtruediv__",
+            "pyo3::class::number::PyNumberRTruedivProtocolImpl",
+        ),
+        PyMethod::coexist(
+            "__rfloordiv__",
+            "pyo3::class::number::PyNumberRFloordivProtocolImpl",
+        ),
+        PyMethod::coexist("__rmod__", "pyo3::class::number::PyNumberRModProtocolImpl"),
+        PyMethod::coexist(
+            "__rdivmod__",
+            "pyo3::class::number::PyNumberRDivmodProtocolImpl",
+        ),
+        PyMethod::coexist("__rpow__", "pyo3::class::number::PyNumberRPowProtocolImpl"),
+        PyMethod::coexist(
+            "__rlshift__",
+            "pyo3::class::number::PyNumberRLShiftProtocolImpl",
+        ),
+        PyMethod::coexist(
+            "__rrshift__",
+            "pyo3::class::number::PyNumberRRShiftProtocolImpl",
+        ),
+        PyMethod::coexist("__rand__", "pyo3::class::number::PyNumberRAndProtocolImpl"),
+        PyMethod::coexist("__rxor__", "pyo3::class::number::PyNumberRXorProtocolImpl"),
+        PyMethod::coexist("__ror__", "pyo3::class::number::PyNumberROrProtocolImpl"),
+        PyMethod::new(
+            "__complex__",
+            "pyo3::class::number::PyNumberComplexProtocolImpl",
+        ),
+        PyMethod::new(
+            "__round__",
+            "pyo3::class::number::PyNumberRoundProtocolImpl",
+        ),
     ],
 };
diff --git a/pyo3-derive-backend/src/func.rs b/pyo3-derive-backend/src/func.rs
index c8e3c83b..fbc48cb7 100644
--- a/pyo3-derive-backend/src/func.rs
+++ b/pyo3-derive-backend/src/func.rs
@@ -69,7 +69,7 @@ impl MethodProto {
     }
 }
 
-pub fn impl_method_proto(
+pub(crate) fn impl_method_proto(
     cls: &syn::Type,
     sig: &mut syn::Signature,
     meth: &MethodProto,
diff --git a/pyo3-derive-backend/src/method.rs b/pyo3-derive-backend/src/method.rs
index 20b06c61..aa02a90b 100644
--- a/pyo3-derive-backend/src/method.rs
+++ b/pyo3-derive-backend/src/method.rs
@@ -63,7 +63,7 @@ impl<'a> FnSpec<'a> {
         let is_mut = self
             .self_
             .expect("impl_borrow_self is called for non-self fn");
-        crate::utils::borrow_self(is_mut, true)
+        crate::utils::borrow_self(is_mut)
     }
 
     /// Parser function signature and function attributes
diff --git a/pyo3-derive-backend/src/module.rs b/pyo3-derive-backend/src/module.rs
index bd9d601a..d6180c5b 100644
--- a/pyo3-derive-backend/src/module.rs
+++ b/pyo3-derive-backend/src/module.rs
@@ -221,13 +221,15 @@ fn function_c_wrapper(name: &Ident, spec: &method::FnSpec<'_>) -> TokenStream {
             const _LOCATION: &'static str = concat!(stringify!(#name), "()");
 
             let _py = pyo3::Python::assume_gil_acquired();
-            let _pool = pyo3::GILPool::new(_py);
-            let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
-            let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
+            pyo3::run_callback(_py, || {
+                let _pool = pyo3::GILPool::new(_py);
+                let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
+                let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
 
-            #body
+                #body
 
-            pyo3::callback::cb_obj_convert(_py, _result)
+                pyo3::callback::convert(_py, _result)
+            })
         }
     }
 }
diff --git a/pyo3-derive-backend/src/pyclass.rs b/pyo3-derive-backend/src/pyclass.rs
index 23697a76..5c92a75b 100644
--- a/pyo3-derive-backend/src/pyclass.rs
+++ b/pyo3-derive-backend/src/pyclass.rs
@@ -459,7 +459,6 @@ fn impl_descriptors(
         .collect::<syn::Result<_>>()?;
 
     Ok(quote! {
-
         pyo3::inventory::submit! {
             #![crate = pyo3] {
                 type ClsInventory = <#cls as pyo3::class::methods::PyMethodsInventoryDispatch>::InventoryType;
diff --git a/pyo3-derive-backend/src/pymethod.rs b/pyo3-derive-backend/src/pymethod.rs
index d590d2b2..729384c0 100644
--- a/pyo3-derive-backend/src/pymethod.rs
+++ b/pyo3-derive-backend/src/pymethod.rs
@@ -83,10 +83,7 @@ pub fn impl_wrap_pyslf(
     };
     let slf = quote! {
         let _cell = _py.from_borrowed_ptr::<pyo3::PyCell<#cls>>(_slf);
-        let _slf: #self_ty = match std::convert::TryFrom::try_from(_cell) {
-            Ok(_slf) => _slf,
-            Err(e) => return pyo3::PyErr::from(e).restore_and_null(_py),
-        };
+        let _slf: #self_ty = std::convert::TryFrom::try_from(_cell)?;
     };
     impl_wrap_common(cls, spec, noargs, slf, body)
 }
@@ -109,13 +106,11 @@ fn impl_wrap_common(
                 const _LOCATION: &'static str = concat!(
                     stringify!(#cls), ".", stringify!(#python_name), "()");
                 let _py = pyo3::Python::assume_gil_acquired();
-                let _pool = pyo3::GILPool::new(_py);
-                #slf
-                let _result = {
-                    pyo3::derive_utils::IntoPyResult::into_py_result(#body)
-                };
-
-                pyo3::callback::cb_obj_convert(_py, _result)
+                pyo3::run_callback(_py, || {
+                    let _pool = pyo3::GILPool::new(_py);
+                    #slf
+                    pyo3::callback::convert(_py, #body)
+                })
             }
         }
     } else {
@@ -130,14 +125,16 @@ fn impl_wrap_common(
                 const _LOCATION: &'static str = concat!(
                     stringify!(#cls), ".", stringify!(#python_name), "()");
                 let _py = pyo3::Python::assume_gil_acquired();
-                let _pool = pyo3::GILPool::new(_py);
-                #slf
-                let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
-                let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
+                pyo3::run_callback(_py, || {
+                    let _pool = pyo3::GILPool::new(_py);
+                    #slf
+                    let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
+                    let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
 
-                #body
+                    #body
 
-                pyo3::callback::cb_obj_convert(_py, _result)
+                    pyo3::callback::convert(_py, _result)
+                })
             }
         }
     }
@@ -159,15 +156,17 @@ pub fn impl_proto_wrap(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
         {
             const _LOCATION: &'static str = concat!(stringify!(#cls),".",stringify!(#python_name),"()");
             let _py = pyo3::Python::assume_gil_acquired();
-            let _pool = pyo3::GILPool::new(_py);
-            let _slf = _py.from_borrowed_ptr::<pyo3::PyCell<#cls>>(_slf);
-            #borrow_self
-            let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
-            let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
+            pyo3::run_callback(_py, || {
+                let _pool = pyo3::GILPool::new(_py);
+                let _slf = _py.from_borrowed_ptr::<pyo3::PyCell<#cls>>(_slf);
+                #borrow_self
+                let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
+                let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
 
-            #body
+                #body
 
-            pyo3::callback::cb_obj_convert(_py, _result)
+                pyo3::callback::convert(_py, _result)
+            })
         }
     }
 }
@@ -195,16 +194,16 @@ pub fn impl_wrap_new(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
 
             const _LOCATION: &'static str = concat!(stringify!(#cls),".",stringify!(#python_name),"()");
             let _py = pyo3::Python::assume_gil_acquired();
-            let _pool = pyo3::GILPool::new(_py);
-            let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
-            let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
+            pyo3::run_callback(_py, || {
+                let _pool = pyo3::GILPool::new(_py);
+                let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
+                let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
 
-            #body
+                #body
 
-            match _result.and_then(|init| pyo3::PyClassInitializer::from(init).create_cell(_py)) {
-                Ok(slf) => slf as _,
-                Err(e) => e.restore_and_null(_py),
-            }
+                let cell = pyo3::PyClassInitializer::from(_result?).create_cell(_py)?;
+                Ok(cell as *mut pyo3::ffi::PyObject)
+            })
         }
     }
 }
@@ -227,14 +226,16 @@ pub fn impl_wrap_class(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
         {
             const _LOCATION: &'static str = concat!(stringify!(#cls),".",stringify!(#python_name),"()");
             let _py = pyo3::Python::assume_gil_acquired();
-            let _pool = pyo3::GILPool::new(_py);
-            let _cls = pyo3::types::PyType::from_type_ptr(_py, _cls as *mut pyo3::ffi::PyTypeObject);
-            let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
-            let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
+            pyo3::run_callback(_py, || {
+                let _pool = pyo3::GILPool::new(_py);
+                let _cls = pyo3::types::PyType::from_type_ptr(_py, _cls as *mut pyo3::ffi::PyTypeObject);
+                let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
+                let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
 
-            #body
+                #body
 
-            pyo3::callback::cb_obj_convert(_py, _result)
+                pyo3::callback::convert(_py, _result)
+            })
         }
     }
 }
@@ -257,13 +258,15 @@ pub fn impl_wrap_static(cls: &syn::Type, spec: &FnSpec<'_>) -> TokenStream {
         {
             const _LOCATION: &'static str = concat!(stringify!(#cls),".",stringify!(#python_name),"()");
             let _py = pyo3::Python::assume_gil_acquired();
-            let _pool = pyo3::GILPool::new(_py);
-            let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
-            let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
+            pyo3::run_callback(_py, || {
+                let _pool = pyo3::GILPool::new(_py);
+                let _args = _py.from_borrowed_ptr::<pyo3::types::PyTuple>(_args);
+                let _kwargs: Option<&pyo3::types::PyDict> = _py.from_borrowed_ptr_or_opt(_kwargs);
 
-            #body
+                #body
 
-            pyo3::callback::cb_obj_convert(_py, _result)
+                pyo3::callback::convert(_py, _result)
+            })
         }
     }
 }
@@ -305,7 +308,7 @@ pub(crate) fn impl_wrap_getter(
         PropertyType::Function(spec) => (spec.python_name.clone(), impl_call_getter(&spec)?),
     };
 
-    let borrow_self = crate::utils::borrow_self(false, true);
+    let borrow_self = crate::utils::borrow_self(false);
     Ok(quote! {
         unsafe extern "C" fn __wrap(
             _slf: *mut pyo3::ffi::PyObject, _: *mut ::std::os::raw::c_void) -> *mut pyo3::ffi::PyObject
@@ -313,16 +316,12 @@ pub(crate) fn impl_wrap_getter(
             const _LOCATION: &'static str = concat!(stringify!(#cls),".",stringify!(#python_name),"()");
 
             let _py = pyo3::Python::assume_gil_acquired();
-            let _pool = pyo3::GILPool::new(_py);
-            let _slf = _py.from_borrowed_ptr::<pyo3::PyCell<#cls>>(_slf);
-            #borrow_self
-
-            let result = pyo3::derive_utils::IntoPyResult::into_py_result(#getter_impl);
-
-            match result {
-                Ok(val) => pyo3::IntoPyPointer::into_ptr(pyo3::IntoPy::<PyObject>::into_py(val, _py)),
-                Err(e) => e.restore_and_null(_py),
-            }
+            pyo3::run_callback(_py, || {
+                let _pool = pyo3::GILPool::new(_py);
+                let _slf = _py.from_borrowed_ptr::<pyo3::PyCell<#cls>>(_slf);
+                #borrow_self
+                pyo3::callback::convert(_py, #getter_impl)
+            })
         }
     })
 }
@@ -344,9 +343,9 @@ fn impl_call_setter(spec: &FnSpec) -> syn::Result<TokenStream> {
 
     let name = &spec.name;
     let fncall = if py_arg.is_some() {
-        quote!(pyo3::derive_utils::IntoPyResult::into_py_result(_slf.#name(_py, _val)))
+        quote!(pyo3::derive_utils::IntoPyResult::into_py_result(_slf.#name(_py, _val))?;)
     } else {
-        quote!(pyo3::derive_utils::IntoPyResult::into_py_result(_slf.#name(_val)))
+        quote!(pyo3::derive_utils::IntoPyResult::into_py_result(_slf.#name(_val))?;)
     };
 
     Ok(fncall)
@@ -360,12 +359,12 @@ pub(crate) fn impl_wrap_setter(
     let (python_name, setter_impl) = match property_type {
         PropertyType::Descriptor(field) => {
             let name = field.ident.as_ref().unwrap();
-            (name.unraw(), quote!({ _slf.#name = _val; Ok(()) }))
+            (name.unraw(), quote!(_slf.#name = _val;))
         }
         PropertyType::Function(spec) => (spec.python_name.clone(), impl_call_setter(&spec)?),
     };
 
-    let borrow_self = crate::utils::borrow_self(true, false);
+    let borrow_self = crate::utils::borrow_self(true);
     Ok(quote! {
         #[allow(unused_mut)]
         unsafe extern "C" fn __wrap(
@@ -374,21 +373,14 @@ pub(crate) fn impl_wrap_setter(
         {
             const _LOCATION: &'static str = concat!(stringify!(#cls),".",stringify!(#python_name),"()");
             let _py = pyo3::Python::assume_gil_acquired();
-            let _pool = pyo3::GILPool::new(_py);
-            let _slf = _py.from_borrowed_ptr::<pyo3::PyCell<#cls>>(_slf);
-            #borrow_self
-            let _value = _py.from_borrowed_ptr(_value);
-
-            let _result = match pyo3::FromPyObject::extract(_value) {
-                Ok(_val) => {
-                    #setter_impl
-                }
-                Err(e) => Err(e)
-            };
-            match _result {
-                Ok(_) => 0,
-                Err(e) => e.restore_and_minus1(_py),
-            }
+            pyo3::run_callback(_py, || {
+                let _pool = pyo3::GILPool::new(_py);
+                let _slf = _py.from_borrowed_ptr::<pyo3::PyCell<#cls>>(_slf);
+                #borrow_self
+                let _value = _py.from_borrowed_ptr::<pyo3::types::PyAny>(_value);
+                let _val = pyo3::FromPyObject::extract(_value)?;
+                pyo3::callback::convert(_py, {#setter_impl})
+            })
         }
     })
 }
@@ -462,22 +454,19 @@ fn impl_arg_params_(spec: &FnSpec<'_>, body: TokenStream, into_result: TokenStre
         let mut _args = _args;
         let mut _kwargs = _kwargs;
 
-        // Workaround to use the question mark operator without rewriting everything
-        let _result = (|| {
-            let (_args, _kwargs) = pyo3::derive_utils::parse_fn_args(
-                Some(_LOCATION),
-                PARAMS,
-                _args,
-                _kwargs,
-                #accept_args,
-                #accept_kwargs,
-                &mut output
-            )?;
+        let (_args, _kwargs) = pyo3::derive_utils::parse_fn_args(
+            Some(_LOCATION),
+            PARAMS,
+            _args,
+            _kwargs,
+            #accept_args,
+            #accept_kwargs,
+            &mut output
+        )?;
 
-            #(#param_conversion)*
+        #(#param_conversion)*
 
-            #into_result(#body)
-        })();
+        let _result = #into_result(#body);
     }
 }
 
diff --git a/pyo3-derive-backend/src/pyproto.rs b/pyo3-derive-backend/src/pyproto.rs
index 5cd32c6e..8f0d9330 100644
--- a/pyo3-derive-backend/src/pyproto.rs
+++ b/pyo3-derive-backend/src/pyproto.rs
@@ -63,39 +63,40 @@ fn impl_proto_impl(
 
     for iimpl in impls.iter_mut() {
         if let syn::ImplItem::Method(ref mut met) = iimpl {
-            for m in proto.methods {
-                if met.sig.ident == m.name() {
-                    impl_method_proto(ty, &mut met.sig, m).to_tokens(&mut tokens);
-                }
+            if let Some(m) = proto.get_proto(&met.sig.ident) {
+                impl_method_proto(ty, &mut met.sig, m).to_tokens(&mut tokens);
             }
-            for m in proto.py_methods {
-                if met.sig.ident == m.name {
-                    let name = &met.sig.ident;
-                    let proto: syn::Path = syn::parse_str(m.proto).unwrap();
+            if let Some(m) = proto.get_method(&met.sig.ident) {
+                let name = &met.sig.ident;
+                let proto: syn::Path = syn::parse_str(m.proto).unwrap();
 
-                    let fn_spec = match FnSpec::parse(&met.sig, &mut met.attrs, false) {
-                        Ok(fn_spec) => fn_spec,
-                        Err(err) => return err.to_compile_error(),
-                    };
-                    let meth = pymethod::impl_proto_wrap(ty, &fn_spec);
+                let fn_spec = match FnSpec::parse(&met.sig, &mut met.attrs, false) {
+                    Ok(fn_spec) => fn_spec,
+                    Err(err) => return err.to_compile_error(),
+                };
+                let meth = pymethod::impl_proto_wrap(ty, &fn_spec);
+                let coexist = if m.can_coexist {
+                    quote!(pyo3::ffi::METH_COEXIST)
+                } else {
+                    quote!(0)
+                };
+                py_methods.push(quote! {
+                    impl #proto for #ty
+                    {
+                        #[inline]
+                        fn #name() -> Option<pyo3::class::methods::PyMethodDef> {
+                            #meth
 
-                    py_methods.push(quote! {
-                        impl #proto for #ty
-                        {
-                            #[inline]
-                            fn #name() -> Option<pyo3::class::methods::PyMethodDef> {
-                                #meth
-
-                                Some(pyo3::class::PyMethodDef {
-                                    ml_name: stringify!(#name),
-                                    ml_meth: pyo3::class::PyMethodType::PyCFunctionWithKeywords(__wrap),
-                                    ml_flags: pyo3::ffi::METH_VARARGS | pyo3::ffi::METH_KEYWORDS,
-                                    ml_doc: ""
-                                })
-                            }
+                            Some(pyo3::class::PyMethodDef {
+                                ml_name: stringify!(#name),
+                                ml_meth: pyo3::class::PyMethodType::PyCFunctionWithKeywords(__wrap),
+                                // We need METH_COEXIST here to prevent __add__  from overriding __radd__
+                                ml_flags: pyo3::ffi::METH_VARARGS | pyo3::ffi::METH_KEYWORDS | #coexist,
+                                ml_doc: ""
+                            })
                         }
-                    });
-                }
+                    }
+                });
             }
         }
     }
diff --git a/pyo3-derive-backend/src/utils.rs b/pyo3-derive-backend/src/utils.rs
index 32b66b25..d051a634 100644
--- a/pyo3-derive-backend/src/utils.rs
+++ b/pyo3-derive-backend/src/utils.rs
@@ -4,25 +4,14 @@ use proc_macro2::TokenStream;
 use quote::quote;
 use std::fmt::Display;
 
-pub(crate) fn borrow_self(is_mut: bool, return_null: bool) -> TokenStream {
-    let ret = if return_null {
-        quote! { restore_and_null }
-    } else {
-        quote! { restore_and_minus1 }
-    };
+pub(crate) fn borrow_self(is_mut: bool) -> TokenStream {
     if is_mut {
         quote! {
-            let mut _slf = match _slf.try_borrow_mut() {
-                Ok(ref_) => ref_,
-                Err(e) => return pyo3::PyErr::from(e).#ret(_py),
-            };
+            let mut _slf = _slf.try_borrow_mut()?;
         }
     } else {
         quote! {
-            let _slf = match _slf.try_borrow() {
-                Ok(ref_) => ref_,
-                Err(e) => return pyo3::PyErr::from(e).#ret(_py),
-            };
+            let _slf = _slf.try_borrow()?;
         }
     }
 }
diff --git a/src/callback.rs b/src/callback.rs
index edeca66c..68603d30 100644
--- a/src/callback.rs
+++ b/src/callback.rs
@@ -7,89 +7,91 @@ use crate::exceptions::OverflowError;
 use crate::ffi::{self, Py_hash_t};
 use crate::IntoPyPointer;
 use crate::{IntoPy, PyObject, Python};
+use std::isize;
 use std::os::raw::c_int;
-use std::{isize, ptr};
+
+/// A type which can be the return type of a python C-API callback
+pub trait PyCallbackOutput: Copy {
+    /// The error value to return to python if the callback raised an exception
+    const ERR_VALUE: Self;
+}
+
+impl PyCallbackOutput for *mut ffi::PyObject {
+    const ERR_VALUE: Self = std::ptr::null_mut();
+}
+
+impl PyCallbackOutput for libc::c_int {
+    const ERR_VALUE: Self = -1;
+}
+
+impl PyCallbackOutput for ffi::Py_ssize_t {
+    const ERR_VALUE: Self = -1;
+}
+
+impl PyCallbackOutput for () {
+    const ERR_VALUE: Self = ();
+}
 
 /// Convert the result of callback function into the appropriate return value.
-///
-/// Used by PyO3 macros.
-pub trait CallbackConverter {
-    type Source;
-    type Result: Copy;
-    const ERR_VALUE: Self::Result;
+pub trait IntoPyCallbackOutput<Target> {
+    fn convert(self, py: Python) -> PyResult<Target>;
+}
 
-    fn convert(s: Self::Source, py: Python) -> Self::Result;
-
-    #[inline]
-    fn convert_result(py: Python, value: PyResult<Self::Source>) -> Self::Result {
-        match value {
-            Ok(val) => Self::convert(val, py),
-            Err(e) => {
-                e.restore(py);
-                Self::ERR_VALUE
-            }
-        }
+impl<T, U> IntoPyCallbackOutput<U> for PyResult<T>
+where
+    T: IntoPyCallbackOutput<U>,
+{
+    fn convert(self, py: Python) -> PyResult<U> {
+        self.and_then(|t| t.convert(py))
     }
 }
 
-pub struct PyObjectCallbackConverter<T>(pub std::marker::PhantomData<T>);
-
-impl<T> CallbackConverter for PyObjectCallbackConverter<T>
+impl<T> IntoPyCallbackOutput<*mut ffi::PyObject> for T
 where
     T: IntoPy<PyObject>,
 {
-    type Source = T;
-    type Result = *mut ffi::PyObject;
-    const ERR_VALUE: Self::Result = ptr::null_mut();
-
-    fn convert(s: Self::Source, py: Python) -> Self::Result {
-        s.into_py(py).into_ptr()
+    fn convert(self, py: Python) -> PyResult<*mut ffi::PyObject> {
+        Ok(self.into_py(py).into_ptr())
     }
 }
 
-pub struct BoolCallbackConverter;
+impl IntoPyCallbackOutput<Self> for *mut ffi::PyObject {
+    fn convert(self, _: Python) -> PyResult<Self> {
+        Ok(self)
+    }
+}
 
-impl CallbackConverter for BoolCallbackConverter {
-    type Source = bool;
-    type Result = c_int;
-    const ERR_VALUE: Self::Result = -1;
+impl IntoPyCallbackOutput<libc::c_int> for () {
+    fn convert(self, _: Python) -> PyResult<libc::c_int> {
+        Ok(0)
+    }
+}
 
+impl IntoPyCallbackOutput<libc::c_int> for bool {
+    fn convert(self, _: Python) -> PyResult<libc::c_int> {
+        Ok(self as c_int)
+    }
+}
+
+impl IntoPyCallbackOutput<()> for () {
+    fn convert(self, _: Python) -> PyResult<()> {
+        Ok(())
+    }
+}
+
+pub struct LenCallbackOutput(pub usize);
+
+impl IntoPyCallbackOutput<ffi::Py_ssize_t> for LenCallbackOutput {
     #[inline]
-    fn convert(s: Self::Source, _py: Python) -> Self::Result {
-        s as c_int
-    }
-}
-
-pub struct LenResultConverter;
-
-impl CallbackConverter for LenResultConverter {
-    type Source = usize;
-    type Result = isize;
-    const ERR_VALUE: Self::Result = -1;
-
-    fn convert(val: Self::Source, py: Python) -> Self::Result {
-        if val <= (isize::MAX as usize) {
-            val as isize
+    fn convert(self, _py: Python) -> PyResult<ffi::Py_ssize_t> {
+        if self.0 <= (isize::MAX as usize) {
+            Ok(self.0 as isize)
         } else {
-            OverflowError::py_err(()).restore(py);
-            -1
+            Err(OverflowError::py_err(()))
         }
     }
 }
 
-pub struct UnitCallbackConverter;
-
-impl CallbackConverter for UnitCallbackConverter {
-    type Source = ();
-    type Result = c_int;
-    const ERR_VALUE: Self::Result = -1;
-
-    #[inline]
-    fn convert(_s: Self::Source, _py: Python) -> Self::Result {
-        0
-    }
-}
-
 pub trait WrappingCastTo<T> {
     fn wrapping_cast(self) -> T;
 }
@@ -115,50 +117,49 @@ wrapping_cast!(i32, Py_hash_t);
 wrapping_cast!(isize, Py_hash_t);
 wrapping_cast!(i64, Py_hash_t);
 
-pub struct HashConverter<T>(pub std::marker::PhantomData<T>);
+pub struct HashCallbackOutput<T>(pub T);
 
-impl<T> CallbackConverter for HashConverter<T>
+impl<T> IntoPyCallbackOutput<Py_hash_t> for HashCallbackOutput<T>
 where
     T: WrappingCastTo<Py_hash_t>,
 {
-    type Source = T;
-    type Result = Py_hash_t;
-    const ERR_VALUE: Self::Result = -1;
-
     #[inline]
-    fn convert(val: T, _py: Python) -> Py_hash_t {
-        let hash = val.wrapping_cast();
+    fn convert(self, _py: Python) -> PyResult<Py_hash_t> {
+        let hash = self.0.wrapping_cast();
         if hash == -1 {
-            -2
+            Ok(-2)
         } else {
-            hash
+            Ok(hash)
         }
     }
 }
 
-// Short hands methods for macros
+#[doc(hidden)]
 #[inline]
-pub fn cb_convert<C, T>(_c: C, py: Python, value: PyResult<T>) -> C::Result
+pub fn convert<T, U>(py: Python, value: T) -> PyResult<U>
 where
-    C: CallbackConverter<Source = T>,
+    T: IntoPyCallbackOutput<U>,
 {
-    C::convert_result(py, value)
+    value.convert(py)
 }
 
-// Same as cb_convert(PyObjectCallbackConverter<T>, py, value)
+#[doc(hidden)]
 #[inline]
-pub fn cb_obj_convert<T: IntoPy<PyObject>>(
-    py: Python,
-    value: PyResult<T>,
-) -> <PyObjectCallbackConverter<T> as CallbackConverter>::Result {
-    PyObjectCallbackConverter::<T>::convert_result(py, value)
+pub fn callback_error<T>() -> T
+where
+    T: PyCallbackOutput,
+{
+    T::ERR_VALUE
 }
 
-#[inline]
-pub unsafe fn cb_err<C>(_c: C, py: Python, err: impl Into<crate::PyErr>) -> C::Result
+#[doc(hidden)]
+pub fn run_callback<T, F>(py: Python, callback: F) -> T
 where
-    C: CallbackConverter,
+    F: FnOnce() -> PyResult<T>,
+    T: PyCallbackOutput,
 {
-    err.into().restore(py);
-    C::ERR_VALUE
+    callback().unwrap_or_else(|e| {
+        e.restore(py);
+        T::ERR_VALUE
+    })
 }
diff --git a/src/class/basic.rs b/src/class/basic.rs
index 722d5a31..2f1a16b5 100644
--- a/src/class/basic.rs
+++ b/src/class/basic.rs
@@ -8,11 +8,11 @@
 //! Parts of the documentation are copied from the respective methods from the
 //! [typeobj docs](https://docs.python.org/3/c-api/typeobj.html)
 
-use crate::callback::{BoolCallbackConverter, HashConverter, PyObjectCallbackConverter};
+use crate::callback::HashCallbackOutput;
 use crate::class::methods::PyMethodDef;
 use crate::{
-    exceptions, ffi, FromPyObject, IntoPy, IntoPyPointer, ObjectProtocol, PyAny, PyClass, PyErr,
-    PyObject, PyResult, Python,
+    callback, exceptions, ffi, run_callback, FromPyObject, GILPool, IntoPy, ObjectProtocol, PyAny,
+    PyCell, PyClass, PyErr, PyObject, PyResult, Python,
 };
 use std::os::raw::c_int;
 
@@ -219,27 +219,23 @@ where
             T: for<'p> PyObjectGetAttrProtocol<'p>,
         {
             let py = Python::assume_gil_acquired();
-            let _pool = crate::GILPool::new(py);
+            run_callback(py, || {
+                let _pool = GILPool::new(py);
 
-            // Behave like python's __getattr__ (as opposed to __getattribute__) and check
-            // for existing fields and methods first
-            let existing = ffi::PyObject_GenericGetAttr(slf, arg);
-            if existing.is_null() {
-                // PyObject_HasAttr also tries to get an object and clears the error if it fails
-                ffi::PyErr_Clear();
-            } else {
-                return existing;
-            }
+                // Behave like python's __getattr__ (as opposed to __getattribute__) and check
+                // for existing fields and methods first
+                let existing = ffi::PyObject_GenericGetAttr(slf, arg);
+                if existing.is_null() {
+                    // PyObject_HasAttr also tries to get an object and clears the error if it fails
+                    ffi::PyErr_Clear();
+                } else {
+                    return Ok(existing);
+                }
 
-            let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
-            let arg = py.from_borrowed_ptr::<crate::PyAny>(arg);
-            call_ref_with_converter!(
-                slf,
-                PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData),
-                py,
-                __getattr__,
-                arg
-            )
+                let slf = py.from_borrowed_ptr::<PyCell<T>>(slf);
+                let arg = py.from_borrowed_ptr::<PyAny>(arg);
+                callback::convert(py, call_ref!(slf, __getattr__, arg))
+            })
         }
         Some(wrap::<T>)
     }
@@ -357,11 +353,7 @@ where
     T: for<'p> PyObjectStrProtocol<'p>,
 {
     fn tp_str() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyObjectStrProtocol,
-            T::__str__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyObjectStrProtocol, T::__str__)
     }
 }
 
@@ -381,11 +373,7 @@ where
     T: for<'p> PyObjectReprProtocol<'p>,
 {
     fn tp_repr() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyObjectReprProtocol,
-            T::__repr__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyObjectReprProtocol, T::__repr__)
     }
 }
 
@@ -447,8 +435,8 @@ where
         py_unary_func!(
             PyObjectHashProtocol,
             T::__hash__,
-            HashConverter::<isize>(std::marker::PhantomData),
-            ffi::Py_hash_t
+            ffi::Py_hash_t,
+            HashCallbackOutput
         )
     }
 }
@@ -469,12 +457,7 @@ where
     T: for<'p> PyObjectBoolProtocol<'p>,
 {
     fn nb_bool() -> Option<ffi::inquiry> {
-        py_unary_func!(
-            PyObjectBoolProtocol,
-            T::__bool__,
-            BoolCallbackConverter,
-            c_int
-        )
+        py_unary_func!(PyObjectBoolProtocol, T::__bool__, c_int)
     }
 }
 
@@ -503,26 +486,17 @@ where
             T: for<'p> PyObjectRichcmpProtocol<'p>,
         {
             let py = Python::assume_gil_acquired();
-            let _pool = crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
-            let arg = py.from_borrowed_ptr::<PyAny>(arg);
+            run_callback(py, || {
+                let _pool = GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
+                let arg = py.from_borrowed_ptr::<PyAny>(arg);
 
-            match slf.try_borrow() {
-                Ok(borrowed_slf) => {
-                    let res = match extract_op(op) {
-                        Ok(op) => match arg.extract() {
-                            Ok(arg) => borrowed_slf.__richcmp__(arg, op).into(),
-                            Err(e) => Err(e),
-                        },
-                        Err(e) => Err(e),
-                    };
-                    match res {
-                        Ok(val) => val.into_py(py).into_ptr(),
-                        Err(e) => e.restore_and_null(py),
-                    }
-                }
-                Err(e) => PyErr::from(e).restore_and_null(py),
-            }
+                let borrowed_slf = slf.try_borrow()?;
+                let op = extract_op(op)?;
+                let arg = arg.extract()?;
+                let result = borrowed_slf.__richcmp__(arg, op).into();
+                callback::convert(py, result)
+            })
         }
         Some(wrap::<T>)
     }
diff --git a/src/class/buffer.rs b/src/class/buffer.rs
index 54e496a1..2177974f 100644
--- a/src/class/buffer.rs
+++ b/src/class/buffer.rs
@@ -4,9 +4,9 @@
 //!
 //! For more information check [buffer protocol](https://docs.python.org/3/c-api/buffer.html)
 //! c-api
-use crate::callback::UnitCallbackConverter;
 use crate::err::PyResult;
-use crate::{ffi, PyClass, PyRefMut};
+use crate::gil::GILPool;
+use crate::{callback, ffi, run_callback, PyCell, PyClass, PyRefMut, Python};
 use std::os::raw::c_int;
 
 /// Buffer protocol interface
@@ -91,14 +91,13 @@ where
         where
             T: for<'p> PyBufferGetBufferProtocol<'p>,
         {
-            let py = crate::Python::assume_gil_acquired();
-            let _pool = crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
-            let result = slf
-                .try_borrow_mut()
-                .map_err(crate::PyErr::from)
-                .and_then(|slf_mut| T::bf_getbuffer(slf_mut, arg1, arg2).into());
-            crate::callback::cb_convert(UnitCallbackConverter, py, result)
+            let py = Python::assume_gil_acquired();
+            run_callback(py, || {
+                let _pool = GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<PyCell<T>>(slf);
+                let result = T::bf_getbuffer(slf.try_borrow_mut()?, arg1, arg2).into();
+                callback::convert(py, result)
+            })
         }
         Some(wrap::<T>)
     }
@@ -127,14 +126,13 @@ where
         where
             T: for<'p> PyBufferReleaseBufferProtocol<'p>,
         {
-            let py = crate::Python::assume_gil_acquired();
-            let _pool = crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
-            let result = slf
-                .try_borrow_mut()
-                .map_err(crate::PyErr::from)
-                .and_then(|slf_mut| T::bf_releasebuffer(slf_mut, arg1).into());
-            crate::callback::cb_convert(UnitCallbackConverter, py, result);
+            let py = Python::assume_gil_acquired();
+            run_callback(py, || {
+                let _pool = GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
+                let result = T::bf_releasebuffer(slf.try_borrow_mut()?, arg1).into();
+                crate::callback::convert(py, result)
+            })
         }
         Some(wrap::<T>)
     }
diff --git a/src/class/descr.rs b/src/class/descr.rs
index 61c2e51c..fb36b72a 100644
--- a/src/class/descr.rs
+++ b/src/class/descr.rs
@@ -5,7 +5,6 @@
 //! [Python information](
 //! https://docs.python.org/3/reference/datamodel.html#implementing-descriptors)
 
-use crate::callback::{PyObjectCallbackConverter, UnitCallbackConverter};
 use crate::class::methods::PyMethodDef;
 use crate::err::PyResult;
 use crate::types::{PyAny, PyType};
@@ -84,11 +83,7 @@ where
     T: for<'p> PyDescrGetProtocol<'p>,
 {
     fn tp_descr_get() -> Option<ffi::descrgetfunc> {
-        py_ternary_func!(
-            PyDescrGetProtocol,
-            T::__get__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_ternary_func!(PyDescrGetProtocol, T::__get__)
     }
 }
 
@@ -108,7 +103,7 @@ where
     T: for<'p> PyDescrSetProtocol<'p>,
 {
     fn tp_descr_set() -> Option<ffi::descrsetfunc> {
-        py_ternary_func!(PyDescrSetProtocol, T::__set__, UnitCallbackConverter, c_int)
+        py_ternary_func!(PyDescrSetProtocol, T::__set__, c_int)
     }
 }
 
diff --git a/src/class/gc.rs b/src/class/gc.rs
index b8638808..e6fe3cf7 100644
--- a/src/class/gc.rs
+++ b/src/class/gc.rs
@@ -98,9 +98,14 @@ where
                 arg,
                 _py: py,
             };
-            match slf.borrow().__traverse__(visit) {
-                Ok(()) => 0,
-                Err(PyTraverseError(code)) => code,
+
+            if let Ok(borrow) = slf.try_borrow() {
+                match borrow.__traverse__(visit) {
+                    Ok(()) => 0,
+                    Err(PyTraverseError(code)) => code,
+                }
+            } else {
+                0
             }
         }
 
diff --git a/src/class/iter.rs b/src/class/iter.rs
index 8d7522e3..0d011d2e 100644
--- a/src/class/iter.rs
+++ b/src/class/iter.rs
@@ -2,10 +2,9 @@
 //! Python Iterator Interface.
 //! Trait and support implementation for implementing iterators
 
-use crate::callback::{CallbackConverter, PyObjectCallbackConverter};
+use crate::callback::IntoPyCallbackOutput;
 use crate::err::PyResult;
 use crate::{ffi, IntoPy, IntoPyPointer, PyClass, PyObject, PyRefMut, Python};
-use std::ptr;
 
 /// Python Iterator Interface.
 ///
@@ -77,11 +76,7 @@ where
 {
     #[inline]
     fn tp_iter() -> Option<ffi::getiterfunc> {
-        py_unary_refmut_func!(
-            PyIterIterProtocol,
-            T::__iter__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_refmut_func!(PyIterIterProtocol, T::__iter__)
     }
 }
 
@@ -104,31 +99,20 @@ where
 {
     #[inline]
     fn tp_iternext() -> Option<ffi::iternextfunc> {
-        py_unary_refmut_func!(
-            PyIterNextProtocol,
-            T::__next__,
-            IterNextConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_refmut_func!(PyIterNextProtocol, T::__next__, IterNextConverter)
     }
 }
 
-struct IterNextConverter<T>(std::marker::PhantomData<T>);
+struct IterNextConverter<T>(Option<T>);
 
-impl<T> CallbackConverter for IterNextConverter<T>
+impl<T> IntoPyCallbackOutput<*mut ffi::PyObject> for IterNextConverter<T>
 where
     T: IntoPy<PyObject>,
 {
-    type Source = Option<T>;
-    type Result = *mut ffi::PyObject;
-    const ERR_VALUE: Self::Result = ptr::null_mut();
-
-    fn convert(val: Self::Source, py: Python) -> Self::Result {
-        match val {
-            Some(val) => val.into_py(py).into_ptr(),
-            None => unsafe {
-                ffi::PyErr_SetNone(ffi::PyExc_StopIteration);
-                ptr::null_mut()
-            },
+    fn convert(self, py: Python) -> PyResult<*mut ffi::PyObject> {
+        match self.0 {
+            Some(val) => Ok(val.into_py(py).into_ptr()),
+            None => Err(crate::exceptions::StopIteration::py_err(())),
         }
     }
 }
diff --git a/src/class/macros.rs b/src/class/macros.rs
index e304ca8c..8d26cd03 100644
--- a/src/class/macros.rs
+++ b/src/class/macros.rs
@@ -3,51 +3,44 @@
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_unary_func {
-    ($trait:ident, $class:ident :: $f:ident, $conv: expr) => {
-        py_unary_func!($trait, $class::$f, $conv, *mut $crate::ffi::PyObject);
-    };
-    // Use call_ref! by default
-    ($trait:ident, $class:ident :: $f:ident, $conv: expr, $ret_type:ty) => {
-        py_unary_func!(
-            $trait,
-            $class::$f,
-            $conv,
-            $ret_type,
-            call_ref_with_converter
-        );
-    };
-    ($trait: ident,
-     $class:ident :: $f:ident,
-     $conv: expr,
-     $ret_type: ty,
-     $call: ident
-    ) => {{
+    ($trait: ident, $class:ident :: $f:ident, $call:ident, $ret_type: ty  $(, $conv:expr)?) => {{
         unsafe extern "C" fn wrap<T>(slf: *mut $crate::ffi::PyObject) -> $ret_type
         where
             T: for<'p> $trait<'p>,
         {
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            $call!(slf, $conv, py, $f)
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
+                $crate::callback::convert(py, $call!(slf, $f)$(.map($conv))?)
+            })
         }
         Some(wrap::<$class>)
     }};
+    // Use call_ref! by default
+    ($trait:ident, $class:ident :: $f:ident, $ret_type:ty $(, $conv:expr)?) => {
+        py_unary_func!($trait, $class::$f, call_ref, $ret_type $(, $conv)?);
+    };
+    ($trait:ident, $class:ident :: $f:ident $(, $conv:expr)?) => {
+        py_unary_func!($trait, $class::$f, call_ref, *mut $crate::ffi::PyObject $(, $conv)?);
+    };
 }
 
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_unary_refmut_func {
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {{
+    ($trait:ident, $class:ident :: $f:ident $(, $conv:expr)?) => {{
         unsafe extern "C" fn wrap<T>(slf: *mut $crate::ffi::PyObject) -> *mut $crate::ffi::PyObject
         where
             T: for<'p> $trait<'p>,
         {
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let res = $class::$f(slf.borrow_mut()).into();
-            $crate::callback::cb_convert($conv, py, res)
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
+                let res = $class::$f(slf.borrow_mut()).into();
+                $crate::callback::convert(py, res $(.map($conv))?)
+            })
         }
         Some(wrap::<$class>)
     }};
@@ -56,51 +49,48 @@ macro_rules! py_unary_refmut_func {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_len_func {
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {{
-        unsafe extern "C" fn wrap<T>(slf: *mut $crate::ffi::PyObject) -> $crate::ffi::Py_ssize_t
-        where
-            T: for<'p> $trait<'p>,
-        {
-            let py = Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let result = call_ref!(slf, $f);
-            $crate::callback::cb_convert($conv, py, result)
-        }
-        Some(wrap::<$class>)
-    }};
+    ($trait:ident, $class:ident :: $f:ident) => {
+        py_unary_func!(
+            $trait,
+            $class::$f,
+            $crate::ffi::Py_ssize_t,
+            $crate::callback::LenCallbackOutput
+        )
+    };
 }
 
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_binary_func {
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {
-        py_binary_func!($trait, $class::$f, $conv, *mut $crate::ffi::PyObject)
-    };
     // Use call_ref! by default
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $return:ty) => {{
-        py_binary_func!($trait, $class::$f, $conv, $return, call_ref_with_converter)
-    }};
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $return:ty, $call:ident) => {{
+    ($trait:ident, $class:ident :: $f:ident, $return:ty, $call:ident $(, $conv:expr)?) => {{
         unsafe extern "C" fn wrap<T>(slf: *mut ffi::PyObject, arg: *mut ffi::PyObject) -> $return
         where
             T: for<'p> $trait<'p>,
         {
             use $crate::ObjectProtocol;
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let arg = py.from_borrowed_ptr::<$crate::PyAny>(arg);
-            $call!(slf, $conv, py, $f, arg)
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
+                let arg = py.from_borrowed_ptr::<$crate::PyAny>(arg);
+                $crate::callback::convert(py, $call!(slf, $f, arg)$(.map($conv))?)
+            })
         }
         Some(wrap::<$class>)
     }};
+    ($trait:ident, $class:ident :: $f:ident, $return:ty $(, $conv:expr)?) => {
+        py_binary_func!($trait, $class::$f, $return, call_ref $(, $conv)?)
+    };
+    ($trait:ident, $class:ident :: $f:ident $(, $conv:expr)?) => {
+        py_binary_func!($trait, $class::$f, *mut $crate::ffi::PyObject $(, $conv)?)
+    };
 }
 
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_binary_num_func {
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {{
+    ($trait:ident, $class:ident :: $f:ident) => {{
         unsafe extern "C" fn wrap<T>(
             lhs: *mut ffi::PyObject,
             rhs: *mut ffi::PyObject,
@@ -110,18 +100,42 @@ macro_rules! py_binary_num_func {
         {
             use $crate::ObjectProtocol;
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let lhs = py.from_borrowed_ptr::<$crate::PyAny>(lhs);
-            let rhs = py.from_borrowed_ptr::<$crate::PyAny>(rhs);
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let lhs = py.from_borrowed_ptr::<$crate::PyAny>(lhs);
+                let rhs = py.from_borrowed_ptr::<$crate::PyAny>(rhs);
 
-            let result = match lhs.extract() {
-                Ok(lhs) => match rhs.extract() {
-                    Ok(rhs) => $class::$f(lhs, rhs).into(),
-                    Err(e) => Err(e.into()),
-                },
-                Err(e) => Err(e.into()),
-            };
-            $crate::callback::cb_convert($conv, py, result)
+                let result = $class::$f(lhs.extract()?, rhs.extract()?).into();
+                $crate::callback::convert(py, result)
+            })
+        }
+        Some(wrap::<$class>)
+    }};
+}
+
+#[macro_export]
+#[doc(hidden)]
+macro_rules! py_binary_reverse_num_func {
+    ($trait:ident, $class:ident :: $f:ident) => {{
+        unsafe extern "C" fn wrap<T>(
+            lhs: *mut ffi::PyObject,
+            rhs: *mut ffi::PyObject,
+        ) -> *mut $crate::ffi::PyObject
+        where
+            T: for<'p> $trait<'p>,
+        {
+            use $crate::ObjectProtocol;
+            let py = $crate::Python::assume_gil_acquired();
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                // Swap lhs <-> rhs
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(rhs);
+                let arg = py.from_borrowed_ptr::<$crate::PyAny>(lhs);
+                $crate::callback::convert(
+                    py,
+                    $class::$f(&*slf.try_borrow()?, arg.extract()?).into(),
+                )
+            })
         }
         Some(wrap::<$class>)
     }};
@@ -142,17 +156,14 @@ macro_rules! py_binary_self_func {
             use $crate::ObjectProtocol;
 
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf_ = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let arg = py.from_borrowed_ptr::<$crate::PyAny>(arg);
-            let result = call_mut!(slf_, $f, arg);
-            match result {
-                Ok(_) => {
-                    ffi::Py_INCREF(slf);
-                    slf
-                }
-                Err(e) => e.restore_and_null(py),
-            }
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf_ = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
+                let arg = py.from_borrowed_ptr::<$crate::PyAny>(arg);
+                call_mut!(slf_, $f, arg)?;
+                ffi::Py_INCREF(slf);
+                Ok(slf)
+            })
         }
         Some(wrap::<$class>)
     }};
@@ -162,15 +173,10 @@ macro_rules! py_binary_self_func {
 #[doc(hidden)]
 macro_rules! py_ssizearg_func {
     // Use call_ref! by default
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {
-        py_ssizearg_func!(
-            $trait,
-            $class::$f,
-            $conv,
-            call_ref_with_converter
-        )
+    ($trait:ident, $class:ident :: $f:ident) => {
+        py_ssizearg_func!($trait, $class::$f, call_ref)
     };
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $call:ident) => {{
+    ($trait:ident, $class:ident :: $f:ident, $call:ident) => {{
         unsafe extern "C" fn wrap<T>(
             slf: *mut ffi::PyObject,
             arg: $crate::ffi::Py_ssize_t,
@@ -179,9 +185,11 @@ macro_rules! py_ssizearg_func {
             T: for<'p> $trait<'p>,
         {
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            $call!(slf, $conv, py, $f ;arg.into())
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
+                $crate::callback::convert(py, $call!(slf, $f; arg.into()))
+            })
         }
         Some(wrap::<$class>)
     }};
@@ -190,10 +198,7 @@ macro_rules! py_ssizearg_func {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_ternary_func {
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {
-        py_ternary_func!($trait, $class::$f, $conv, *mut $crate::ffi::PyObject);
-    };
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr, $return_type:ty) => {{
+    ($trait:ident, $class:ident :: $f:ident, $return_type:ty) => {{
         unsafe extern "C" fn wrap<T>(
             slf: *mut $crate::ffi::PyObject,
             arg1: *mut $crate::ffi::PyObject,
@@ -205,22 +210,31 @@ macro_rules! py_ternary_func {
             use $crate::ObjectProtocol;
 
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let arg1 = py.from_borrowed_ptr::<$crate::PyAny>(arg1);
-            let arg2 = py.from_borrowed_ptr::<$crate::PyAny>(arg2);
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
+                let arg1 = py
+                    .from_borrowed_ptr::<$crate::types::PyAny>(arg1)
+                    .extract()?;
+                let arg2 = py
+                    .from_borrowed_ptr::<$crate::types::PyAny>(arg2)
+                    .extract()?;
 
-            call_ref_with_converter!(slf, $conv, py, $f, arg1, arg2)
+                $crate::callback::convert(py, slf.try_borrow()?.$f(arg1, arg2).into())
+            })
         }
 
         Some(wrap::<T>)
     }};
+    ($trait:ident, $class:ident :: $f:ident) => {
+        py_ternary_func!($trait, $class::$f, *mut $crate::ffi::PyObject);
+    };
 }
 
 #[macro_export]
 #[doc(hidden)]
 macro_rules! py_ternary_num_func {
-    ($trait:ident, $class:ident :: $f:ident, $conv:expr) => {{
+    ($trait:ident, $class:ident :: $f:ident) => {{
         unsafe extern "C" fn wrap<T>(
             arg1: *mut $crate::ffi::PyObject,
             arg2: *mut $crate::ffi::PyObject,
@@ -232,22 +246,21 @@ macro_rules! py_ternary_num_func {
             use $crate::ObjectProtocol;
 
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let arg1 = py.from_borrowed_ptr::<$crate::PyAny>(arg1);
-            let arg2 = py.from_borrowed_ptr::<$crate::PyAny>(arg2);
-            let arg3 = py.from_borrowed_ptr::<$crate::PyAny>(arg3);
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let arg1 = py
+                    .from_borrowed_ptr::<$crate::types::PyAny>(arg1)
+                    .extract()?;
+                let arg2 = py
+                    .from_borrowed_ptr::<$crate::types::PyAny>(arg2)
+                    .extract()?;
+                let arg3 = py
+                    .from_borrowed_ptr::<$crate::types::PyAny>(arg3)
+                    .extract()?;
 
-            let result = match arg1.extract() {
-                Ok(arg1) => match arg2.extract() {
-                    Ok(arg2) => match arg3.extract() {
-                        Ok(arg3) => $class::$f(arg1, arg2, arg3).into(),
-                        Err(e) => Err(e.into()),
-                    },
-                    Err(e) => Err(e.into()),
-                },
-                Err(e) => Err(e.into()),
-            };
-            $crate::callback::cb_convert($conv, py, result)
+                let result = $class::$f(arg1, arg2, arg3).into();
+                $crate::callback::convert(py, result)
+            })
         }
 
         Some(wrap::<T>)
@@ -256,12 +269,43 @@ macro_rules! py_ternary_num_func {
 
 #[macro_export]
 #[doc(hidden)]
-macro_rules! py_ternary_self_func {
+macro_rules! py_ternary_reverse_num_func {
+    ($trait:ident, $class:ident :: $f:ident) => {{
+        unsafe extern "C" fn wrap<T>(
+            arg1: *mut $crate::ffi::PyObject,
+            arg2: *mut $crate::ffi::PyObject,
+            arg3: *mut $crate::ffi::PyObject,
+        ) -> *mut $crate::ffi::PyObject
+        where
+            T: for<'p> $trait<'p>,
+        {
+            use $crate::ObjectProtocol;
+            let py = $crate::Python::assume_gil_acquired();
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                // Swap lhs <-> rhs
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<T>>(arg2);
+                let slf = slf.try_borrow()?;
+                let arg1 = py.from_borrowed_ptr::<$crate::PyAny>(arg1);
+                let arg2 = py.from_borrowed_ptr::<$crate::PyAny>(arg3);
+                let result = $class::$f(&*slf, arg1.extract()?, arg2.extract()?).into();
+                $crate::callback::convert(py, result)
+            })
+        }
+        Some(wrap::<$class>)
+    }};
+}
+
+// NOTE(kngwyu): Somehow __ipow__ causes SIGSEGV in Python < 3.8 when we extract arg2,
+// so we ignore it. It's the same as what CPython does.
+#[macro_export]
+#[doc(hidden)]
+macro_rules! py_dummy_ternary_self_func {
     ($trait:ident, $class:ident :: $f:ident) => {{
         unsafe extern "C" fn wrap<T>(
             slf: *mut $crate::ffi::PyObject,
             arg1: *mut $crate::ffi::PyObject,
-            arg2: *mut $crate::ffi::PyObject,
+            _arg2: *mut $crate::ffi::PyObject,
         ) -> *mut $crate::ffi::PyObject
         where
             T: for<'p> $trait<'p>,
@@ -269,17 +313,16 @@ macro_rules! py_ternary_self_func {
             use $crate::ObjectProtocol;
 
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf_cell = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
-            let arg1 = py.from_borrowed_ptr::<$crate::PyAny>(arg1);
-            let arg2 = py.from_borrowed_ptr::<$crate::PyAny>(arg2);
-            let result = call_mut!(slf_cell, $f, arg1, arg2);
-            match result {
-                Ok(_) => slf,
-                Err(e) => e.restore_and_null(py),
-            }
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf_cell = py.from_borrowed_ptr::<$crate::PyCell<T>>(slf);
+                let arg1 = py.from_borrowed_ptr::<$crate::PyAny>(arg1);
+                call_mut!(slf_cell, $f, arg1)?;
+                ffi::Py_INCREF(slf);
+                Ok(slf)
+            })
         }
-        Some(wrap::<T>)
+        Some(wrap::<$class>)
     }};
 }
 
@@ -296,25 +339,23 @@ macro_rules! py_func_set {
             use $crate::ObjectProtocol;
 
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<$generic>>(slf);
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<$generic>>(slf);
 
-            let result = if value.is_null() {
-                Err($crate::PyErr::new::<exceptions::NotImplementedError, _>(
-                    format!(
-                        "Subscript deletion not supported by {:?}",
-                        stringify!($generic)
-                    ),
-                ))
-            } else {
-                let name = py.from_borrowed_ptr::<$crate::PyAny>(name);
-                let value = py.from_borrowed_ptr::<$crate::PyAny>(value);
-                call_mut!(slf, $fn_set, name, value)
-            };
-            match result {
-                Ok(_) => 0,
-                Err(e) => e.restore_and_minus1(py),
-            }
+                if value.is_null() {
+                    Err($crate::PyErr::new::<exceptions::NotImplementedError, _>(
+                        format!(
+                            "Subscript deletion not supported by {:?}",
+                            stringify!($generic)
+                        ),
+                    ))
+                } else {
+                    let name = py.from_borrowed_ptr::<$crate::PyAny>(name);
+                    let value = py.from_borrowed_ptr::<$crate::PyAny>(value);
+                    crate::callback::convert(py, call_mut!(slf, $fn_set, name, value))
+                }
+            })
         }
 
         Some(wrap::<$generic>)
@@ -334,22 +375,21 @@ macro_rules! py_func_del {
             use $crate::ObjectProtocol;
 
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
 
-            let result = if value.is_null() {
-                let slf = py.from_borrowed_ptr::<$crate::PyCell<U>>(slf);
-                let name = py.from_borrowed_ptr::<$crate::PyAny>(name);
-
-                call_mut!(slf, $fn_del, name)
-            } else {
-                Err(PyErr::new::<exceptions::NotImplementedError, _>(
-                    "Subscript assignment not supported",
-                ))
-            };
-            match result {
-                Ok(_) => 0,
-                Err(e) => e.restore_and_minus1(py),
-            }
+                if value.is_null() {
+                    let slf = py.from_borrowed_ptr::<$crate::PyCell<U>>(slf);
+                    let name = py
+                        .from_borrowed_ptr::<$crate::types::PyAny>(name)
+                        .extract()?;
+                    $crate::callback::convert(py, slf.try_borrow_mut()?.$fn_del(name).into())
+                } else {
+                    Err(PyErr::new::<exceptions::NotImplementedError, _>(
+                        "Subscript assignment not supported",
+                    ))
+                }
+            })
         }
 
         Some(wrap::<$generic>)
@@ -369,71 +409,43 @@ macro_rules! py_func_set_del {
             use $crate::ObjectProtocol;
 
             let py = $crate::Python::assume_gil_acquired();
-            let _pool = $crate::GILPool::new(py);
-            let slf = py.from_borrowed_ptr::<$crate::PyCell<$generic>>(slf);
-            let name = py.from_borrowed_ptr::<$crate::PyAny>(name);
+            $crate::run_callback(py, || {
+                let _pool = $crate::GILPool::new(py);
+                let slf = py.from_borrowed_ptr::<$crate::PyCell<$generic>>(slf);
+                let name = py.from_borrowed_ptr::<$crate::PyAny>(name);
 
-            let result = if value.is_null() {
-                call_mut!(slf, $fn_del, name)
-            } else {
-                let value = py.from_borrowed_ptr::<$crate::PyAny>(value);
-                call_mut!(slf, $fn_set, name, value)
-            };
-            match result {
-                Ok(_) => 0,
-                Err(e) => e.restore_and_minus1(py),
-            }
+                let result = if value.is_null() {
+                    call_mut!(slf, $fn_del, name)
+                } else {
+                    let value = py.from_borrowed_ptr::<$crate::PyAny>(value);
+                    call_mut!(slf, $fn_set, name, value)
+                };
+                $crate::callback::convert(py, result)
+            })
         }
         Some(wrap::<$generic>)
     }};
 }
 
 macro_rules! _call_impl {
-    ($slf: ident, $fn: ident $(; $args: expr)*) => { $slf.$fn($($args,)*).into() };
-    ($slf: ident, $fn: ident, $raw_arg: expr $(,$raw_args: expr)* $(; $args: expr)*) => {
-        match $raw_arg.extract() {
-            Ok(arg) => _call_impl!($slf, $fn $(,$raw_args)* $(;$args)* ;arg),
-            Err(e) => Err(e.into()),
-        }
+    ($slf: expr, $fn: ident $(; $args: expr)*) => {
+        $slf.$fn($($args,)*).into()
+    };
+    ($slf: expr, $fn: ident, $raw_arg: expr $(,$raw_args: expr)* $(; $args: expr)*) => {
+        _call_impl!($slf, $fn $(,$raw_args)* $(;$args)* ;$raw_arg.extract()?)
     };
 }
 
 /// Call `slf.try_borrow()?.$fn(...)`
 macro_rules! call_ref {
     ($slf: expr, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
-        match $slf.try_borrow() {
-            Ok(slf) => _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*),
-            Err(e) => Err(e.into()),
-        }
-    };
-}
-
-/// Call `slf.try_borrow()?.$fn(...)` and returns the result using the given CallbackConverter
-macro_rules! call_ref_with_converter {
-    ($slf: expr, $conv: expr, $py: ident, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
-        match $slf.try_borrow() {
-            Ok(slf) => $crate::callback::cb_convert($conv, $py, _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*)),
-            Err(e) => $crate::callback::cb_err($conv, $py, e)
-        }
+        _call_impl!($slf.try_borrow()?, $fn $(,$raw_args)* $(;$args)*)
     };
 }
 
 /// Call `slf.try_borrow_mut()?.$fn(...)`
 macro_rules! call_mut {
     ($slf: expr, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
-        match $slf.try_borrow_mut() {
-            Ok(mut slf) => _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*),
-            Err(e) => Err(e.into()),
-        }
-    };
-}
-
-/// Call `slf.try_borrow_mut()?.$fn(...)` and returns the result using the given CallbackConverter
-macro_rules! call_mut_with_converter {
-    ($slf: expr, $conv: expr, $py: ident, $fn: ident $(,$raw_args: expr)* $(; $args: expr)*) => {
-        match $slf.try_borrow_mut() {
-            Ok(mut slf) => $crate::callback::cb_convert($conv, $py, _call_impl!(slf, $fn $(,$raw_args)* $(;$args)*)),
-            Err(e) => $crate::callback::cb_err($conv, $py, e)
-        }
+        _call_impl!($slf.try_borrow_mut()?, $fn $(,$raw_args)* $(;$args)*)
     };
 }
diff --git a/src/class/mapping.rs b/src/class/mapping.rs
index 39373134..4d82a9dd 100644
--- a/src/class/mapping.rs
+++ b/src/class/mapping.rs
@@ -3,10 +3,9 @@
 //! Python Mapping Interface
 //! Trait and support implementation for implementing mapping support
 
-use crate::callback::{LenResultConverter, PyObjectCallbackConverter};
 use crate::class::methods::PyMethodDef;
 use crate::err::{PyErr, PyResult};
-use crate::{exceptions, ffi, FromPyObject, IntoPy, PyClass, PyObject, Python};
+use crate::{exceptions, ffi, FromPyObject, IntoPy, PyClass, PyObject};
 
 /// Mapping interface
 #[allow(unused_variables)]
@@ -141,7 +140,7 @@ where
 {
     #[inline]
     fn mp_length() -> Option<ffi::lenfunc> {
-        py_len_func!(PyMappingLenProtocol, T::__len__, LenResultConverter)
+        py_len_func!(PyMappingLenProtocol, T::__len__)
     }
 }
 
@@ -164,11 +163,7 @@ where
 {
     #[inline]
     fn mp_subscript() -> Option<ffi::binaryfunc> {
-        py_binary_func!(
-            PyMappingGetItemProtocol,
-            T::__getitem__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_func!(PyMappingGetItemProtocol, T::__getitem__)
     }
 }
 
diff --git a/src/class/number.rs b/src/class/number.rs
index 1be54f54..207a5b3a 100644
--- a/src/class/number.rs
+++ b/src/class/number.rs
@@ -3,7 +3,6 @@
 //! Python Number Interface
 //! Trait and support implementation for implementing number protocol
 
-use crate::callback::PyObjectCallbackConverter;
 use crate::class::basic::PyObjectProtocolImpl;
 use crate::class::methods::PyMethodDef;
 use crate::err::PyResult;
@@ -60,7 +59,7 @@ pub trait PyNumberProtocol<'p>: PyClass {
     {
         unimplemented!()
     }
-    fn __pow__(lhs: Self::Left, rhs: Self::Right, modulo: Self::Modulo) -> Self::Result
+    fn __pow__(lhs: Self::Left, rhs: Self::Right, modulo: Option<Self::Modulo>) -> Self::Result
     where
         Self: PyNumberPowProtocol<'p>,
     {
@@ -145,7 +144,7 @@ pub trait PyNumberProtocol<'p>: PyClass {
     {
         unimplemented!()
     }
-    fn __rpow__(&'p self, other: Self::Other) -> Self::Result
+    fn __rpow__(&'p self, other: Self::Other, modulo: Option<Self::Modulo>) -> Self::Result
     where
         Self: PyNumberRPowProtocol<'p>,
     {
@@ -224,7 +223,7 @@ pub trait PyNumberProtocol<'p>: PyClass {
     {
         unimplemented!()
     }
-    fn __ipow__(&'p mut self, other: Self::Other, modulo: Self::Modulo) -> Self::Result
+    fn __ipow__(&'p mut self, other: Self::Other) -> Self::Result
     where
         Self: PyNumberIPowProtocol<'p>,
     {
@@ -304,18 +303,18 @@ pub trait PyNumberProtocol<'p>: PyClass {
     {
         unimplemented!()
     }
-    fn __round__(&'p self) -> Self::Result
-    where
-        Self: PyNumberRoundProtocol<'p>,
-    {
-        unimplemented!()
-    }
     fn __index__(&'p self) -> Self::Result
     where
         Self: PyNumberIndexProtocol<'p>,
     {
         unimplemented!()
     }
+    fn __round__(&'p self, ndigits: Option<Self::NDigits>) -> Self::Result
+    where
+        Self: PyNumberRoundProtocol<'p>,
+    {
+        unimplemented!()
+    }
 }
 
 pub trait PyNumberAddProtocol<'p>: PyNumberProtocol<'p> {
@@ -544,7 +543,6 @@ pub trait PyNumberIDivmodProtocol<'p>: PyNumberProtocol<'p> {
 
 pub trait PyNumberIPowProtocol<'p>: PyNumberProtocol<'p> {
     type Other: FromPyObject<'p>;
-    type Modulo: FromPyObject<'p>;
     type Result: Into<PyResult<()>>;
 }
 
@@ -610,6 +608,7 @@ pub trait PyNumberFloatProtocol<'p>: PyNumberProtocol<'p> {
 
 pub trait PyNumberRoundProtocol<'p>: PyNumberProtocol<'p> {
     type Success: IntoPy<PyObject>;
+    type NDigits: FromPyObject<'p>;
     type Result: Into<PyResult<Self::Success>>;
 }
 
@@ -647,22 +646,22 @@ where
 {
     fn tp_as_number() -> Option<ffi::PyNumberMethods> {
         Some(ffi::PyNumberMethods {
-            nb_add: Self::nb_add(),
-            nb_subtract: Self::nb_subtract(),
-            nb_multiply: Self::nb_multiply(),
+            nb_add: Self::nb_add().or_else(Self::nb_add_fallback),
+            nb_subtract: Self::nb_subtract().or_else(Self::nb_sub_fallback),
+            nb_multiply: Self::nb_multiply().or_else(Self::nb_mul_fallback),
             nb_remainder: Self::nb_remainder(),
-            nb_divmod: Self::nb_divmod(),
-            nb_power: Self::nb_power(),
+            nb_divmod: Self::nb_divmod().or_else(Self::nb_divmod_fallback),
+            nb_power: Self::nb_power().or_else(Self::nb_pow_fallback),
             nb_negative: Self::nb_negative(),
             nb_positive: Self::nb_positive(),
             nb_absolute: Self::nb_absolute(),
             nb_bool: <Self as PyObjectProtocolImpl>::nb_bool_fn(),
             nb_invert: Self::nb_invert(),
-            nb_lshift: Self::nb_lshift(),
-            nb_rshift: Self::nb_rshift(),
-            nb_and: Self::nb_and(),
-            nb_xor: Self::nb_xor(),
-            nb_or: Self::nb_or(),
+            nb_lshift: Self::nb_lshift().or_else(Self::nb_lshift_fallback),
+            nb_rshift: Self::nb_rshift().or_else(Self::nb_rshift_fallback),
+            nb_and: Self::nb_and().or_else(Self::nb_and_fallback),
+            nb_xor: Self::nb_xor().or_else(Self::nb_xor_fallback),
+            nb_or: Self::nb_or().or_else(Self::nb_or_fallback),
             nb_int: Self::nb_int(),
             nb_reserved: ::std::ptr::null_mut(),
             nb_float: Self::nb_float(),
@@ -676,12 +675,12 @@ where
             nb_inplace_and: Self::nb_inplace_and(),
             nb_inplace_xor: Self::nb_inplace_xor(),
             nb_inplace_or: Self::nb_inplace_or(),
-            nb_floor_divide: Self::nb_floor_divide(),
-            nb_true_divide: Self::nb_true_divide(),
+            nb_floor_divide: Self::nb_floor_divide().or_else(Self::nb_floordiv_fallback),
+            nb_true_divide: Self::nb_true_divide().or_else(Self::nb_truediv_fallback),
             nb_inplace_floor_divide: Self::nb_inplace_floor_divide(),
             nb_inplace_true_divide: Self::nb_inplace_true_divide(),
             nb_index: Self::nb_index(),
-            nb_matrix_multiply: Self::nb_matrix_multiply(),
+            nb_matrix_multiply: Self::nb_matrix_multiply().or_else(Self::nb_matmul_fallback),
             nb_inplace_matrix_multiply: Self::nb_inplace_matrix_multiply(),
         })
     }
@@ -761,11 +760,7 @@ where
     T: for<'p> PyNumberAddProtocol<'p>,
 {
     fn nb_add() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberAddProtocol,
-            T::__add__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberAddProtocol, T::__add__)
     }
 }
 
@@ -787,11 +782,7 @@ where
     T: for<'p> PyNumberSubProtocol<'p>,
 {
     fn nb_subtract() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberSubProtocol,
-            T::__sub__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberSubProtocol, T::__sub__)
     }
 }
 
@@ -813,11 +804,7 @@ where
     T: for<'p> PyNumberMulProtocol<'p>,
 {
     fn nb_multiply() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberMulProtocol,
-            T::__mul__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberMulProtocol, T::__mul__)
     }
 }
 
@@ -839,11 +826,7 @@ where
     T: for<'p> PyNumberMatmulProtocol<'p>,
 {
     fn nb_matrix_multiply() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberMatmulProtocol,
-            T::__matmul__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberMatmulProtocol, T::__matmul__)
     }
 }
 
@@ -865,11 +848,7 @@ where
     T: for<'p> PyNumberTruedivProtocol<'p>,
 {
     fn nb_true_divide() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberTruedivProtocol,
-            T::__truediv__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberTruedivProtocol, T::__truediv__)
     }
 }
 
@@ -891,11 +870,7 @@ where
     T: for<'p> PyNumberFloordivProtocol<'p>,
 {
     fn nb_floor_divide() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberFloordivProtocol,
-            T::__floordiv__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberFloordivProtocol, T::__floordiv__)
     }
 }
 
@@ -917,11 +892,7 @@ where
     T: for<'p> PyNumberModProtocol<'p>,
 {
     fn nb_remainder() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberModProtocol,
-            T::__mod__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberModProtocol, T::__mod__)
     }
 }
 
@@ -943,11 +914,7 @@ where
     T: for<'p> PyNumberDivmodProtocol<'p>,
 {
     fn nb_divmod() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberDivmodProtocol,
-            T::__divmod__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberDivmodProtocol, T::__divmod__)
     }
 }
 
@@ -969,11 +936,7 @@ where
     T: for<'p> PyNumberPowProtocol<'p>,
 {
     fn nb_power() -> Option<ffi::ternaryfunc> {
-        py_ternary_num_func!(
-            PyNumberPowProtocol,
-            T::__pow__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_ternary_num_func!(PyNumberPowProtocol, T::__pow__)
     }
 }
 
@@ -995,11 +958,7 @@ where
     T: for<'p> PyNumberLShiftProtocol<'p>,
 {
     fn nb_lshift() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberLShiftProtocol,
-            T::__lshift__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberLShiftProtocol, T::__lshift__)
     }
 }
 
@@ -1021,11 +980,7 @@ where
     T: for<'p> PyNumberRShiftProtocol<'p>,
 {
     fn nb_rshift() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberRShiftProtocol,
-            T::__rshift__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberRShiftProtocol, T::__rshift__)
     }
 }
 
@@ -1047,11 +1002,7 @@ where
     T: for<'p> PyNumberAndProtocol<'p>,
 {
     fn nb_and() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberAndProtocol,
-            T::__and__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberAndProtocol, T::__and__)
     }
 }
 
@@ -1073,11 +1024,7 @@ where
     T: for<'p> PyNumberXorProtocol<'p>,
 {
     fn nb_xor() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberXorProtocol,
-            T::__xor__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberXorProtocol, T::__xor__)
     }
 }
 
@@ -1099,11 +1046,7 @@ where
     T: for<'p> PyNumberOrProtocol<'p>,
 {
     fn nb_or() -> Option<ffi::binaryfunc> {
-        py_binary_num_func!(
-            PyNumberOrProtocol,
-            T::__or__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_num_func!(PyNumberOrProtocol, T::__or__)
     }
 }
 
@@ -1279,7 +1222,7 @@ where
     T: for<'p> PyNumberIPowProtocol<'p>,
 {
     fn nb_inplace_power() -> Option<ffi::ternaryfunc> {
-        py_ternary_self_func!(PyNumberIPowProtocol, T::__ipow__)
+        py_dummy_ternary_self_func!(PyNumberIPowProtocol, T::__ipow__)
     }
 }
 
@@ -1407,14 +1350,64 @@ where
     }
 }
 
-#[doc(hidden)]
-pub trait PyNumberRSubProtocolImpl {
-    fn __rsub__() -> Option<PyMethodDef> {
+// Fallback trait for nb_add
+trait PyNumberAddFallback {
+    fn nb_add_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberAddFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_add_fallback() -> Option<ffi::binaryfunc> {
         None
     }
 }
 
-impl<'p, T> PyNumberRSubProtocolImpl for T where T: PyNumberProtocol<'p> {}
+impl<T> PyNumberAddFallback for T
+where
+    T: for<'p> PyNumberRAddProtocol<'p>,
+{
+    fn nb_add_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRAddProtocol, T::__radd__)
+    }
+}
+
+#[doc(hidden)]
+pub trait PyNumberRSubProtocolImpl {
+    fn __rsub__() -> Option<PyMethodDef>;
+}
+
+impl<'p, T> PyNumberRSubProtocolImpl for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn __rsub__() -> Option<PyMethodDef> {
+        None
+    }
+}
+
+trait PyNumberSubFallback {
+    fn nb_sub_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberSubFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_sub_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberSubFallback for T
+where
+    T: for<'p> PyNumberRSubProtocol<'p>,
+{
+    fn nb_sub_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRSubProtocol, T::__rsub__)
+    }
+}
 
 #[doc(hidden)]
 pub trait PyNumberRMulProtocolImpl {
@@ -1430,6 +1423,28 @@ where
     }
 }
 
+trait PyNumberMulFallback {
+    fn nb_mul_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberMulFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_mul_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberMulFallback for T
+where
+    T: for<'p> PyNumberRMulProtocol<'p>,
+{
+    fn nb_mul_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRMulProtocol, T::__rmul__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRMatmulProtocolImpl {
     fn __rmatmul__() -> Option<PyMethodDef>;
@@ -1444,6 +1459,28 @@ where
     }
 }
 
+trait PyNumberMatmulFallback {
+    fn nb_matmul_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberMatmulFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_matmul_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberMatmulFallback for T
+where
+    T: for<'p> PyNumberRMatmulProtocol<'p>,
+{
+    fn nb_matmul_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRMatmulProtocol, T::__rmatmul__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRTruedivProtocolImpl {
     fn __rtruediv__() -> Option<PyMethodDef>;
@@ -1458,6 +1495,28 @@ where
     }
 }
 
+trait PyNumberTruedivFallback {
+    fn nb_truediv_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberTruedivFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_truediv_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberTruedivFallback for T
+where
+    T: for<'p> PyNumberRTruedivProtocol<'p>,
+{
+    fn nb_truediv_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRTruedivProtocol, T::__rtruediv__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRFloordivProtocolImpl {
     fn __rfloordiv__() -> Option<PyMethodDef>;
@@ -1472,6 +1531,28 @@ where
     }
 }
 
+trait PyNumberFloordivFallback {
+    fn nb_floordiv_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberFloordivFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_floordiv_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberFloordivFallback for T
+where
+    T: for<'p> PyNumberRFloordivProtocol<'p>,
+{
+    fn nb_floordiv_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRFloordivProtocol, T::__rfloordiv__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRModProtocolImpl {
     fn __rmod__() -> Option<PyMethodDef>;
@@ -1486,6 +1567,28 @@ where
     }
 }
 
+trait PyNumberModFallback {
+    fn nb_mod_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberModFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_mod_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberModFallback for T
+where
+    T: for<'p> PyNumberRModProtocol<'p>,
+{
+    fn nb_mod_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRModProtocol, T::__rmod__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRDivmodProtocolImpl {
     fn __rdivmod__() -> Option<PyMethodDef>;
@@ -1500,6 +1603,28 @@ where
     }
 }
 
+trait PyNumberDivmodFallback {
+    fn nb_divmod_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberDivmodFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_divmod_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberDivmodFallback for T
+where
+    T: for<'p> PyNumberRDivmodProtocol<'p>,
+{
+    fn nb_divmod_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRDivmodProtocol, T::__rdivmod__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRPowProtocolImpl {
     fn __rpow__() -> Option<PyMethodDef>;
@@ -1514,6 +1639,28 @@ where
     }
 }
 
+trait PyNumberPowFallback {
+    fn nb_pow_fallback() -> Option<ffi::ternaryfunc>;
+}
+
+impl<'p, T> PyNumberPowFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_pow_fallback() -> Option<ffi::ternaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberPowFallback for T
+where
+    T: for<'p> PyNumberRPowProtocol<'p>,
+{
+    fn nb_pow_fallback() -> Option<ffi::ternaryfunc> {
+        py_ternary_reverse_num_func!(PyNumberRPowProtocol, T::__rpow__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRLShiftProtocolImpl {
     fn __rlshift__() -> Option<PyMethodDef>;
@@ -1528,6 +1675,28 @@ where
     }
 }
 
+trait PyNumberLShiftFallback {
+    fn nb_lshift_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberLShiftFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_lshift_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberLShiftFallback for T
+where
+    T: for<'p> PyNumberRLShiftProtocol<'p>,
+{
+    fn nb_lshift_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRLShiftProtocol, T::__rlshift__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRRShiftProtocolImpl {
     fn __rrshift__() -> Option<PyMethodDef>;
@@ -1542,6 +1711,28 @@ where
     }
 }
 
+trait PyNumberRRshiftFallback {
+    fn nb_rshift_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberRRshiftFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_rshift_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberRRshiftFallback for T
+where
+    T: for<'p> PyNumberRRShiftProtocol<'p>,
+{
+    fn nb_rshift_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRRShiftProtocol, T::__rrshift__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRAndProtocolImpl {
     fn __rand__() -> Option<PyMethodDef>;
@@ -1556,6 +1747,28 @@ where
     }
 }
 
+trait PyNumberAndFallback {
+    fn nb_and_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberAndFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_and_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberAndFallback for T
+where
+    T: for<'p> PyNumberRAndProtocol<'p>,
+{
+    fn nb_and_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRAndProtocol, T::__rand__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberRXorProtocolImpl {
     fn __rxor__() -> Option<PyMethodDef>;
@@ -1570,6 +1783,28 @@ where
     }
 }
 
+trait PyNumberXorFallback {
+    fn nb_xor_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberXorFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_xor_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberXorFallback for T
+where
+    T: for<'p> PyNumberRXorProtocol<'p>,
+{
+    fn nb_xor_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberRXorProtocol, T::__rxor__)
+    }
+}
+
 #[doc(hidden)]
 pub trait PyNumberROrProtocolImpl {
     fn __ror__() -> Option<PyMethodDef>;
@@ -1584,6 +1819,28 @@ where
     }
 }
 
+trait PyNumberOrFallback {
+    fn nb_or_fallback() -> Option<ffi::binaryfunc>;
+}
+
+impl<'p, T> PyNumberOrFallback for T
+where
+    T: PyNumberProtocol<'p>,
+{
+    default fn nb_or_fallback() -> Option<ffi::binaryfunc> {
+        None
+    }
+}
+
+impl<T> PyNumberOrFallback for T
+where
+    T: for<'p> PyNumberROrProtocol<'p>,
+{
+    fn nb_or_fallback() -> Option<ffi::binaryfunc> {
+        py_binary_reverse_num_func!(PyNumberROrProtocol, T::__ror__)
+    }
+}
+
 trait PyNumberNegProtocolImpl {
     fn nb_negative() -> Option<ffi::unaryfunc>;
 }
@@ -1603,11 +1860,7 @@ where
 {
     #[inline]
     fn nb_negative() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyNumberNegProtocol,
-            T::__neg__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyNumberNegProtocol, T::__neg__)
     }
 }
 
@@ -1629,11 +1882,7 @@ where
     T: for<'p> PyNumberPosProtocol<'p>,
 {
     fn nb_positive() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyNumberPosProtocol,
-            T::__pos__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyNumberPosProtocol, T::__pos__)
     }
 }
 
@@ -1655,11 +1904,7 @@ where
     T: for<'p> PyNumberAbsProtocol<'p>,
 {
     fn nb_absolute() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyNumberAbsProtocol,
-            T::__abs__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyNumberAbsProtocol, T::__abs__)
     }
 }
 
@@ -1681,11 +1926,7 @@ where
     T: for<'p> PyNumberInvertProtocol<'p>,
 {
     fn nb_invert() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyNumberInvertProtocol,
-            T::__invert__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyNumberInvertProtocol, T::__invert__)
     }
 }
 
@@ -1707,11 +1948,7 @@ where
     T: for<'p> PyNumberIntProtocol<'p>,
 {
     fn nb_int() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyNumberIntProtocol,
-            T::__int__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyNumberIntProtocol, T::__int__)
     }
 }
 
@@ -1733,11 +1970,7 @@ where
     T: for<'p> PyNumberFloatProtocol<'p>,
 {
     fn nb_float() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyNumberFloatProtocol,
-            T::__float__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyNumberFloatProtocol, T::__float__)
     }
 }
 
@@ -1759,15 +1992,11 @@ where
     T: for<'p> PyNumberIndexProtocol<'p>,
 {
     fn nb_index() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyNumberIndexProtocol,
-            T::__index__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyNumberIndexProtocol, T::__index__)
     }
 }
 
-trait PyNumberComplexProtocolImpl {
+pub trait PyNumberComplexProtocolImpl {
     fn __complex__() -> Option<PyMethodDef>;
 }
 
@@ -1780,7 +2009,7 @@ where
     }
 }
 
-trait PyNumberRoundProtocolImpl {
+pub trait PyNumberRoundProtocolImpl {
     fn __round__() -> Option<PyMethodDef>;
 }
 
diff --git a/src/class/pyasync.rs b/src/class/pyasync.rs
index 3a423ffa..a43dc57c 100644
--- a/src/class/pyasync.rs
+++ b/src/class/pyasync.rs
@@ -8,7 +8,6 @@
 //! [PEP-0492](https://www.python.org/dev/peps/pep-0492/)
 //!
 
-use crate::callback::PyObjectCallbackConverter;
 use crate::class::methods::PyMethodDef;
 use crate::err::PyResult;
 use crate::{ffi, PyClass, PyObject};
@@ -150,11 +149,7 @@ where
 {
     #[inline]
     fn am_await() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyAsyncAwaitProtocol,
-            T::__await__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyAsyncAwaitProtocol, T::__await__)
     }
 }
 
@@ -177,11 +172,7 @@ where
 {
     #[inline]
     fn am_aiter() -> Option<ffi::unaryfunc> {
-        py_unary_func!(
-            PyAsyncAiterProtocol,
-            T::__aiter__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_unary_func!(PyAsyncAiterProtocol, T::__aiter__)
     }
 }
 
@@ -200,30 +191,22 @@ where
 
 mod anext {
     use super::{PyAsyncAnextProtocol, PyAsyncAnextProtocolImpl};
-    use crate::callback::CallbackConverter;
+    use crate::callback::IntoPyCallbackOutput;
+    use crate::err::PyResult;
     use crate::IntoPyPointer;
     use crate::Python;
     use crate::{ffi, IntoPy, PyObject};
-    use std::marker::PhantomData;
-    use std::ptr;
 
-    struct IterANextResultConverter<T>(PhantomData<T>);
+    struct IterANextOutput<T>(Option<T>);
 
-    impl<T> CallbackConverter for IterANextResultConverter<T>
+    impl<T> IntoPyCallbackOutput<*mut ffi::PyObject> for IterANextOutput<T>
     where
         T: IntoPy<PyObject>,
     {
-        type Source = Option<T>;
-        type Result = *mut ffi::PyObject;
-        const ERR_VALUE: Self::Result = ptr::null_mut();
-
-        fn convert(val: Self::Source, py: Python) -> Self::Result {
-            match val {
-                Some(val) => val.into_py(py).into_ptr(),
-                None => unsafe {
-                    ffi::PyErr_SetNone(ffi::PyExc_StopAsyncIteration);
-                    ptr::null_mut()
-                },
+        fn convert(self, py: Python) -> PyResult<*mut ffi::PyObject> {
+            match self.0 {
+                Some(val) => Ok(val.into_py(py).into_ptr()),
+                None => Err(crate::exceptions::StopAsyncIteration::py_err(())),
             }
         }
     }
@@ -237,9 +220,9 @@ mod anext {
             py_unary_func!(
                 PyAsyncAnextProtocol,
                 T::__anext__,
-                IterANextResultConverter::<T::Success>(std::marker::PhantomData),
+                call_mut,
                 *mut crate::ffi::PyObject,
-                call_mut_with_converter
+                IterANextOutput
             )
         }
     }
diff --git a/src/class/sequence.rs b/src/class/sequence.rs
index 498844b7..eeb23600 100644
--- a/src/class/sequence.rs
+++ b/src/class/sequence.rs
@@ -3,10 +3,11 @@
 //! Python Sequence Interface
 //! Trait and support implementation for implementing sequence
 
-use crate::callback::{BoolCallbackConverter, LenResultConverter, PyObjectCallbackConverter};
+use crate::conversion::{FromPyObject, IntoPy};
 use crate::err::{PyErr, PyResult};
+use crate::gil::GILPool;
 use crate::objectprotocol::ObjectProtocol;
-use crate::{exceptions, ffi, FromPyObject, IntoPy, PyAny, PyClass, PyObject, Python};
+use crate::{callback, exceptions, ffi, run_callback, PyAny, PyCell, PyClass, PyObject, Python};
 use std::os::raw::c_int;
 
 /// Sequence interface
@@ -176,7 +177,7 @@ where
     T: for<'p> PySequenceLenProtocol<'p>,
 {
     fn sq_length() -> Option<ffi::lenfunc> {
-        py_len_func!(PySequenceLenProtocol, T::__len__, LenResultConverter)
+        py_len_func!(PySequenceLenProtocol, T::__len__)
     }
 }
 
@@ -198,11 +199,7 @@ where
     T: for<'p> PySequenceGetItemProtocol<'p>,
 {
     fn sq_item() -> Option<ffi::ssizeargfunc> {
-        py_ssizearg_func!(
-            PySequenceGetItemProtocol,
-            T::__getitem__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_ssizearg_func!(PySequenceGetItemProtocol, T::__getitem__)
     }
 }
 
@@ -260,31 +257,23 @@ mod sq_ass_item_impl {
                 T: for<'p> PySequenceSetItemProtocol<'p>,
             {
                 let py = Python::assume_gil_acquired();
-                let _pool = crate::GILPool::new(py);
-                let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
+                run_callback(py, || {
+                    let _pool = GILPool::new(py);
+                    let slf = py.from_borrowed_ptr::<PyCell<T>>(slf);
 
-                if value.is_null() {
-                    return PyErr::new::<exceptions::NotImplementedError, _>(format!(
-                        "Item deletion is not supported by {:?}",
-                        stringify!(T)
-                    ))
-                    .restore_and_minus1(py);
-                }
-
-                let result = match slf.try_borrow_mut() {
-                    Ok(mut slf) => {
-                        let value = py.from_borrowed_ptr::<PyAny>(value);
-                        match value.extract() {
-                            Ok(value) => slf.__setitem__(key.into(), value).into(),
-                            Err(e) => e.into(),
-                        }
+                    if value.is_null() {
+                        return Err(PyErr::new::<exceptions::NotImplementedError, _>(format!(
+                            "Item deletion is not supported by {:?}",
+                            stringify!(T)
+                        )));
                     }
-                    Err(e) => Err(PyErr::from(e)),
-                };
-                match result {
-                    Ok(_) => 0,
-                    Err(e) => e.restore_and_minus1(py),
-                }
+
+                    let mut slf = slf.try_borrow_mut()?;
+                    let value = py.from_borrowed_ptr::<PyAny>(value);
+                    let value = value.extract()?;
+                    let result = slf.__setitem__(key.into(), value).into();
+                    callback::convert(py, result)
+                })
             }
             Some(wrap::<T>)
         }
@@ -317,22 +306,21 @@ mod sq_ass_item_impl {
                 T: for<'p> PySequenceDelItemProtocol<'p>,
             {
                 let py = Python::assume_gil_acquired();
-                let _pool = crate::GILPool::new(py);
-                let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
+                run_callback(py, || {
+                    let _pool = GILPool::new(py);
+                    let slf = py.from_borrowed_ptr::<PyCell<T>>(slf);
 
-                let result = if value.is_null() {
-                    slf.borrow_mut().__delitem__(key.into()).into()
-                } else {
-                    Err(PyErr::new::<exceptions::NotImplementedError, _>(format!(
-                        "Item assignment not supported by {:?}",
-                        stringify!(T)
-                    )))
-                };
+                    let result = if value.is_null() {
+                        slf.borrow_mut().__delitem__(key.into()).into()
+                    } else {
+                        Err(PyErr::new::<exceptions::NotImplementedError, _>(format!(
+                            "Item assignment not supported by {:?}",
+                            stringify!(T)
+                        )))
+                    };
 
-                match result {
-                    Ok(_) => 0,
-                    Err(e) => e.restore_and_minus1(py),
-                }
+                    callback::convert(py, result)
+                })
             }
             Some(wrap::<T>)
         }
@@ -365,25 +353,20 @@ mod sq_ass_item_impl {
                 T: for<'p> PySequenceSetItemProtocol<'p> + for<'p> PySequenceDelItemProtocol<'p>,
             {
                 let py = Python::assume_gil_acquired();
-                let _pool = crate::GILPool::new(py);
-                let slf = py.from_borrowed_ptr::<crate::PyCell<T>>(slf);
+                run_callback(py, || {
+                    let _pool = GILPool::new(py);
+                    let slf = py.from_borrowed_ptr::<PyCell<T>>(slf);
 
-                let result = if value.is_null() {
-                    call_mut!(slf, __delitem__; key.into())
-                } else {
-                    let value = py.from_borrowed_ptr::<PyAny>(value);
-                    match slf.try_borrow_mut() {
-                        Ok(mut slf_) => match value.extract() {
-                            Ok(value) => slf_.__setitem__(key.into(), value).into(),
-                            Err(e) => Err(e),
-                        },
-                        Err(e) => Err(e.into()),
-                    }
-                };
-                match result {
-                    Ok(_) => 0,
-                    Err(e) => e.restore_and_minus1(py),
-                }
+                    let result = if value.is_null() {
+                        call_mut!(slf, __delitem__; key.into())
+                    } else {
+                        let value = py.from_borrowed_ptr::<PyAny>(value);
+                        let mut slf_ = slf.try_borrow_mut()?;
+                        let value = value.extract()?;
+                        slf_.__setitem__(key.into(), value).into()
+                    };
+                    callback::convert(py, result)
+                })
             }
             Some(wrap::<T>)
         }
@@ -408,12 +391,7 @@ where
     T: for<'p> PySequenceContainsProtocol<'p>,
 {
     fn sq_contains() -> Option<ffi::objobjproc> {
-        py_binary_func!(
-            PySequenceContainsProtocol,
-            T::__contains__,
-            BoolCallbackConverter,
-            c_int
-        )
+        py_binary_func!(PySequenceContainsProtocol, T::__contains__, c_int)
     }
 }
 
@@ -435,11 +413,7 @@ where
     T: for<'p> PySequenceConcatProtocol<'p>,
 {
     fn sq_concat() -> Option<ffi::binaryfunc> {
-        py_binary_func!(
-            PySequenceConcatProtocol,
-            T::__concat__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_binary_func!(PySequenceConcatProtocol, T::__concat__)
     }
 }
 
@@ -461,11 +435,7 @@ where
     T: for<'p> PySequenceRepeatProtocol<'p>,
 {
     fn sq_repeat() -> Option<ffi::ssizeargfunc> {
-        py_ssizearg_func!(
-            PySequenceRepeatProtocol,
-            T::__repeat__,
-            PyObjectCallbackConverter::<T::Success>(std::marker::PhantomData)
-        )
+        py_ssizearg_func!(PySequenceRepeatProtocol, T::__repeat__)
     }
 }
 
@@ -490,9 +460,8 @@ where
         py_binary_func!(
             PySequenceInplaceConcatProtocol,
             T::__inplace_concat__,
-            PyObjectCallbackConverter::<T>(std::marker::PhantomData),
-            *mut crate::ffi::PyObject,
-            call_mut_with_converter
+            *mut ffi::PyObject,
+            call_mut
         )
     }
 }
@@ -518,8 +487,7 @@ where
         py_ssizearg_func!(
             PySequenceInplaceRepeatProtocol,
             T::__inplace_repeat__,
-            PyObjectCallbackConverter::<T>(std::marker::PhantomData),
-            call_mut_with_converter
+            call_mut
         )
     }
 }
diff --git a/src/err.rs b/src/err.rs
index cc0dbcd7..2cdfcac3 100644
--- a/src/err.rs
+++ b/src/err.rs
@@ -1,15 +1,12 @@
 // Copyright (c) 2017-present PyO3 Project and Contributors
 
-use crate::instance::Py;
-use crate::object::PyObject;
 use crate::type_object::PyTypeObject;
-use crate::types::{PyAny, PyType};
-use crate::AsPyPointer;
-use crate::IntoPyPointer;
-use crate::Python;
-use crate::{exceptions, IntoPy};
-use crate::{ffi, FromPy};
-use crate::{ToBorrowedObject, ToPyObject};
+use crate::types::PyType;
+use crate::{exceptions, ffi};
+use crate::{
+    AsPyPointer, FromPy, IntoPy, IntoPyPointer, Py, PyAny, PyObject, Python, ToBorrowedObject,
+    ToPyObject,
+};
 use libc::c_int;
 use std::ffi::CString;
 use std::io;
diff --git a/src/ffi/bytesobject.rs b/src/ffi/bytesobject.rs
index b150e3b1..17362e6c 100644
--- a/src/ffi/bytesobject.rs
+++ b/src/ffi/bytesobject.rs
@@ -37,9 +37,9 @@ extern "C" {
     pub fn PyBytes_AsString(arg1: *mut PyObject) -> *mut c_char;
     pub fn PyBytes_Repr(arg1: *mut PyObject, arg2: c_int) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyBytes_Concat")]
-    pub fn PyBytes_Concat(arg1: *mut *mut PyObject, arg2: *mut PyObject) -> ();
+    pub fn PyBytes_Concat(arg1: *mut *mut PyObject, arg2: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyBytes_ConcatAndDel")]
-    pub fn PyBytes_ConcatAndDel(arg1: *mut *mut PyObject, arg2: *mut PyObject) -> ();
+    pub fn PyBytes_ConcatAndDel(arg1: *mut *mut PyObject, arg2: *mut PyObject);
     pub fn PyBytes_DecodeEscape(
         arg1: *const c_char,
         arg2: Py_ssize_t,
diff --git a/src/ffi/ceval.rs b/src/ffi/ceval.rs
index f5265c75..df815ee3 100644
--- a/src/ffi/ceval.rs
+++ b/src/ffi/ceval.rs
@@ -45,7 +45,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPy_MakePendingCalls")]
     pub fn Py_MakePendingCalls() -> c_int;
     #[cfg_attr(PyPy, link_name = "PyPy_SetRecursionLimit")]
-    pub fn Py_SetRecursionLimit(arg1: c_int) -> ();
+    pub fn Py_SetRecursionLimit(arg1: c_int);
     #[cfg_attr(PyPy, link_name = "PyPy_GetRecursionLimit")]
     pub fn Py_GetRecursionLimit() -> c_int;
     fn _Py_CheckRecursiveCall(_where: *mut c_char) -> c_int;
@@ -74,7 +74,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyEval_SaveThread")]
     pub fn PyEval_SaveThread() -> *mut PyThreadState;
     #[cfg_attr(PyPy, link_name = "PyPyEval_RestoreThread")]
-    pub fn PyEval_RestoreThread(arg1: *mut PyThreadState) -> ();
+    pub fn PyEval_RestoreThread(arg1: *mut PyThreadState);
 }
 
 #[cfg(py_sys_config = "WITH_THREAD")]
@@ -83,13 +83,13 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyEval_ThreadsInitialized")]
     pub fn PyEval_ThreadsInitialized() -> c_int;
     #[cfg_attr(PyPy, link_name = "PyPyEval_InitThreads")]
-    pub fn PyEval_InitThreads() -> ();
-    pub fn PyEval_AcquireLock() -> ();
-    pub fn PyEval_ReleaseLock() -> ();
+    pub fn PyEval_InitThreads();
+    pub fn PyEval_AcquireLock();
+    pub fn PyEval_ReleaseLock();
     #[cfg_attr(PyPy, link_name = "PyPyEval_AcquireThread")]
-    pub fn PyEval_AcquireThread(tstate: *mut PyThreadState) -> ();
+    pub fn PyEval_AcquireThread(tstate: *mut PyThreadState);
     #[cfg_attr(PyPy, link_name = "PyPyEval_ReleaseThread")]
-    pub fn PyEval_ReleaseThread(tstate: *mut PyThreadState) -> ();
+    pub fn PyEval_ReleaseThread(tstate: *mut PyThreadState);
     #[cfg(not(Py_3_8))]
-    pub fn PyEval_ReInitThreads() -> ();
+    pub fn PyEval_ReInitThreads();
 }
diff --git a/src/ffi/dictobject.rs b/src/ffi/dictobject.rs
index 046f8896..995e61f4 100644
--- a/src/ffi/dictobject.rs
+++ b/src/ffi/dictobject.rs
@@ -69,6 +69,8 @@ pub unsafe fn PyDictViewSet_Check(op: *mut PyObject) -> c_int {
 extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyDict_New")]
     pub fn PyDict_New() -> *mut PyObject;
+    #[cfg(not(PyPy))]
+    pub fn _PyDict_NewPresized(minused: Py_ssize_t) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyDict_GetItem")]
     pub fn PyDict_GetItem(mp: *mut PyObject, key: *mut PyObject) -> *mut PyObject;
     pub fn PyDict_GetItemWithError(mp: *mut PyObject, key: *mut PyObject) -> *mut PyObject;
@@ -84,7 +86,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyDict_DelItem")]
     pub fn PyDict_DelItem(mp: *mut PyObject, key: *mut PyObject) -> c_int;
     #[cfg_attr(PyPy, link_name = "PyPyDict_Clear")]
-    pub fn PyDict_Clear(mp: *mut PyObject) -> ();
+    pub fn PyDict_Clear(mp: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyDict_Next")]
     pub fn PyDict_Next(
         mp: *mut PyObject,
diff --git a/src/ffi/frameobject.rs b/src/ffi/frameobject.rs
index ff564977..3e459a0b 100644
--- a/src/ffi/frameobject.rs
+++ b/src/ffi/frameobject.rs
@@ -65,17 +65,12 @@ extern "C" {
         locals: *mut PyObject,
     ) -> *mut PyFrameObject;
 
-    pub fn PyFrame_BlockSetup(
-        f: *mut PyFrameObject,
-        _type: c_int,
-        handler: c_int,
-        level: c_int,
-    ) -> ();
+    pub fn PyFrame_BlockSetup(f: *mut PyFrameObject, _type: c_int, handler: c_int, level: c_int);
     pub fn PyFrame_BlockPop(f: *mut PyFrameObject) -> *mut PyTryBlock;
 
-    pub fn PyFrame_LocalsToFast(f: *mut PyFrameObject, clear: c_int) -> ();
+    pub fn PyFrame_LocalsToFast(f: *mut PyFrameObject, clear: c_int);
     pub fn PyFrame_FastToLocalsWithError(f: *mut PyFrameObject) -> c_int;
-    pub fn PyFrame_FastToLocals(f: *mut PyFrameObject) -> ();
+    pub fn PyFrame_FastToLocals(f: *mut PyFrameObject);
 
     pub fn PyFrame_ClearFreeList() -> c_int;
     pub fn PyFrame_GetLineNumber(f: *mut PyFrameObject) -> c_int;
diff --git a/src/ffi/import.rs b/src/ffi/import.rs
index b72c6526..a96bb911 100644
--- a/src/ffi/import.rs
+++ b/src/ffi/import.rs
@@ -68,7 +68,7 @@ extern "C" {
     pub fn PyImport_Import(name: *mut PyObject) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyImport_ReloadModule")]
     pub fn PyImport_ReloadModule(m: *mut PyObject) -> *mut PyObject;
-    pub fn PyImport_Cleanup() -> ();
+    pub fn PyImport_Cleanup();
     pub fn PyImport_ImportFrozenModuleObject(name: *mut PyObject) -> c_int;
     pub fn PyImport_ImportFrozenModule(name: *const c_char) -> c_int;
 
diff --git a/src/ffi/intrcheck.rs b/src/ffi/intrcheck.rs
index 4e1e3088..d4710c3a 100644
--- a/src/ffi/intrcheck.rs
+++ b/src/ffi/intrcheck.rs
@@ -4,7 +4,7 @@ use std::os::raw::c_int;
 extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyOS_InterruptOccurred")]
     pub fn PyOS_InterruptOccurred() -> c_int;
-    pub fn PyOS_InitInterrupts() -> ();
+    pub fn PyOS_InitInterrupts();
     #[cfg_attr(PyPy, link_name = "PyPyOS_AfterFork")]
-    pub fn PyOS_AfterFork() -> ();
+    pub fn PyOS_AfterFork();
 }
diff --git a/src/ffi/object.rs b/src/ffi/object.rs
index 6ceb6d3b..67fd1133 100644
--- a/src/ffi/object.rs
+++ b/src/ffi/object.rs
@@ -165,7 +165,7 @@ mod bufferinfo {
         arg3: c_int,
     ) -> c_int;
     pub type releasebufferproc =
-        unsafe extern "C" fn(arg1: *mut crate::ffi::PyObject, arg2: *mut Py_buffer) -> ();
+        unsafe extern "C" fn(arg1: *mut crate::ffi::PyObject, arg2: *mut Py_buffer);
 
     /// Maximum number of dimensions
     pub const PyBUF_MAX_NDIM: c_int = 64;
@@ -829,9 +829,9 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyCallable_Check")]
     pub fn PyCallable_Check(arg1: *mut PyObject) -> c_int;
     #[cfg_attr(PyPy, link_name = "PyPyObject_ClearWeakRefs")]
-    pub fn PyObject_ClearWeakRefs(arg1: *mut PyObject) -> ();
+    pub fn PyObject_ClearWeakRefs(arg1: *mut PyObject);
     #[cfg(not(Py_LIMITED_API))]
-    pub fn PyObject_CallFinalizer(arg1: *mut PyObject) -> ();
+    pub fn PyObject_CallFinalizer(arg1: *mut PyObject);
     #[cfg(not(Py_LIMITED_API))]
     #[cfg_attr(PyPy, link_name = "PyPyObject_CallFinalizerFromDealloc")]
     pub fn PyObject_CallFinalizerFromDealloc(arg1: *mut PyObject) -> c_int;
@@ -839,7 +839,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyObject_Dir")]
     pub fn PyObject_Dir(arg1: *mut PyObject) -> *mut PyObject;
     pub fn Py_ReprEnter(arg1: *mut PyObject) -> c_int;
-    pub fn Py_ReprLeave(arg1: *mut PyObject) -> ();
+    pub fn Py_ReprLeave(arg1: *mut PyObject);
 }
 
 // Flag bits for printing:
@@ -908,7 +908,7 @@ pub unsafe fn PyType_FastSubclass(t: *mut PyTypeObject, f: c_ulong) -> c_int {
 #[cfg_attr(windows, link(name = "pythonXY"))]
 extern "C" {
     #[cfg_attr(PyPy, link_name = "_PyPy_Dealloc")]
-    pub fn _Py_Dealloc(arg1: *mut PyObject) -> ();
+    pub fn _Py_Dealloc(arg1: *mut PyObject);
 }
 
 // Reference counting macros.
diff --git a/src/ffi/objectabstract.rs b/src/ffi/objectabstract.rs
index a1fb403f..e180d3c3 100644
--- a/src/ffi/objectabstract.rs
+++ b/src/ffi/objectabstract.rs
@@ -132,7 +132,7 @@ extern "C" {
         strides: *mut Py_ssize_t,
         itemsize: c_int,
         fort: c_char,
-    ) -> ();
+    );
     #[cfg_attr(PyPy, link_name = "PyPyBuffer_FillInfo")]
     pub fn PyBuffer_FillInfo(
         view: *mut Py_buffer,
@@ -143,7 +143,7 @@ extern "C" {
         flags: c_int,
     ) -> c_int;
     #[cfg_attr(PyPy, link_name = "PyPyBuffer_Release")]
-    pub fn PyBuffer_Release(view: *mut Py_buffer) -> ();
+    pub fn PyBuffer_Release(view: *mut Py_buffer);
 }
 
 #[cfg_attr(windows, link(name = "pythonXY"))]
diff --git a/src/ffi/objimpl.rs b/src/ffi/objimpl.rs
index 60f192a5..c6f4cf8b 100644
--- a/src/ffi/objimpl.rs
+++ b/src/ffi/objimpl.rs
@@ -11,7 +11,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyObject_Realloc")]
     pub fn PyObject_Realloc(ptr: *mut c_void, new_size: size_t) -> *mut c_void;
     #[cfg_attr(PyPy, link_name = "PyPyObject_Free")]
-    pub fn PyObject_Free(ptr: *mut c_void) -> ();
+    pub fn PyObject_Free(ptr: *mut c_void);
 
     #[cfg(not(Py_LIMITED_API))]
     pub fn _Py_GetAllocatedBlocks() -> Py_ssize_t;
@@ -37,7 +37,7 @@ extern "C" {
 pub struct PyObjectArenaAllocator {
     pub ctx: *mut c_void,
     pub alloc: Option<extern "C" fn(ctx: *mut c_void, size: size_t) -> *mut c_void>,
-    pub free: Option<extern "C" fn(ctx: *mut c_void, ptr: *mut c_void, size: size_t) -> ()>,
+    pub free: Option<extern "C" fn(ctx: *mut c_void, ptr: *mut c_void, size: size_t)>,
 }
 
 #[cfg(not(Py_LIMITED_API))]
@@ -50,8 +50,8 @@ impl Default for PyObjectArenaAllocator {
 #[cfg(not(Py_LIMITED_API))]
 #[cfg_attr(windows, link(name = "pythonXY"))]
 extern "C" {
-    pub fn PyObject_GetArenaAllocator(allocator: *mut PyObjectArenaAllocator) -> ();
-    pub fn PyObject_SetArenaAllocator(allocator: *mut PyObjectArenaAllocator) -> ();
+    pub fn PyObject_GetArenaAllocator(allocator: *mut PyObjectArenaAllocator);
+    pub fn PyObject_SetArenaAllocator(allocator: *mut PyObjectArenaAllocator);
 }
 
 /// Test if a type has a GC head
@@ -84,10 +84,10 @@ extern "C" {
     pub fn _PyObject_GC_New(arg1: *mut PyTypeObject) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "_PyPyObject_GC_NewVar")]
     pub fn _PyObject_GC_NewVar(arg1: *mut PyTypeObject, arg2: Py_ssize_t) -> *mut PyVarObject;
-    pub fn PyObject_GC_Track(arg1: *mut c_void) -> ();
-    pub fn PyObject_GC_UnTrack(arg1: *mut c_void) -> ();
+    pub fn PyObject_GC_Track(arg1: *mut c_void);
+    pub fn PyObject_GC_UnTrack(arg1: *mut c_void);
     #[cfg_attr(PyPy, link_name = "PyPyObject_GC_Del")]
-    pub fn PyObject_GC_Del(arg1: *mut c_void) -> ();
+    pub fn PyObject_GC_Del(arg1: *mut c_void);
 }
 
 /// Test if a type supports weak references
diff --git a/src/ffi/pyerrors.rs b/src/ffi/pyerrors.rs
index 1e81a2e4..2467902f 100644
--- a/src/ffi/pyerrors.rs
+++ b/src/ffi/pyerrors.rs
@@ -9,31 +9,31 @@ use std::os::raw::{c_char, c_int};
 #[cfg_attr(windows, link(name = "pythonXY"))]
 extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyErr_SetNone")]
-    pub fn PyErr_SetNone(arg1: *mut PyObject) -> ();
+    pub fn PyErr_SetNone(arg1: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyErr_SetObject")]
-    pub fn PyErr_SetObject(arg1: *mut PyObject, arg2: *mut PyObject) -> ();
+    pub fn PyErr_SetObject(arg1: *mut PyObject, arg2: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyErr_SetString")]
-    pub fn PyErr_SetString(exception: *mut PyObject, string: *const c_char) -> ();
+    pub fn PyErr_SetString(exception: *mut PyObject, string: *const c_char);
     #[cfg_attr(PyPy, link_name = "PyPyErr_Occurred")]
     pub fn PyErr_Occurred() -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyErr_Clear")]
-    pub fn PyErr_Clear() -> ();
+    pub fn PyErr_Clear();
     #[cfg_attr(PyPy, link_name = "PyPyErr_Fetch")]
     pub fn PyErr_Fetch(
         arg1: *mut *mut PyObject,
         arg2: *mut *mut PyObject,
         arg3: *mut *mut PyObject,
-    ) -> ();
+    );
     #[cfg_attr(PyPy, link_name = "PyPyErr_Restore")]
-    pub fn PyErr_Restore(arg1: *mut PyObject, arg2: *mut PyObject, arg3: *mut PyObject) -> ();
+    pub fn PyErr_Restore(arg1: *mut PyObject, arg2: *mut PyObject, arg3: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyErr_GetExcInfo")]
     pub fn PyErr_GetExcInfo(
         arg1: *mut *mut PyObject,
         arg2: *mut *mut PyObject,
         arg3: *mut *mut PyObject,
-    ) -> ();
+    );
     #[cfg_attr(PyPy, link_name = "PyPyErr_SetExcInfo")]
-    pub fn PyErr_SetExcInfo(arg1: *mut PyObject, arg2: *mut PyObject, arg3: *mut PyObject) -> ();
+    pub fn PyErr_SetExcInfo(arg1: *mut PyObject, arg2: *mut PyObject, arg3: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPy_FatalError")]
     pub fn Py_FatalError(message: *const c_char) -> !;
     #[cfg_attr(PyPy, link_name = "PyPyErr_GivenExceptionMatches")]
@@ -45,7 +45,7 @@ extern "C" {
         arg1: *mut *mut PyObject,
         arg2: *mut *mut PyObject,
         arg3: *mut *mut PyObject,
-    ) -> ();
+    );
     #[cfg_attr(PyPy, link_name = "PyPyException_SetTraceback")]
     pub fn PyException_SetTraceback(arg1: *mut PyObject, arg2: *mut PyObject) -> c_int;
     #[cfg_attr(PyPy, link_name = "PyPyException_GetTraceback")]
@@ -53,11 +53,11 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyException_GetCause")]
     pub fn PyException_GetCause(arg1: *mut PyObject) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyException_SetCause")]
-    pub fn PyException_SetCause(arg1: *mut PyObject, arg2: *mut PyObject) -> ();
+    pub fn PyException_SetCause(arg1: *mut PyObject, arg2: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyException_GetContext")]
     pub fn PyException_GetContext(arg1: *mut PyObject) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyException_SetContext")]
-    pub fn PyException_SetContext(arg1: *mut PyObject, arg2: *mut PyObject) -> ();
+    pub fn PyException_SetContext(arg1: *mut PyObject, arg2: *mut PyObject);
 }
 
 #[inline]
@@ -277,8 +277,8 @@ extern "C" {
         arg3: *mut PyObject,
     ) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyErr_BadInternalCall")]
-    pub fn PyErr_BadInternalCall() -> ();
-    pub fn _PyErr_BadInternalCall(filename: *const c_char, lineno: c_int) -> ();
+    pub fn PyErr_BadInternalCall();
+    pub fn _PyErr_BadInternalCall(filename: *const c_char, lineno: c_int);
     #[cfg_attr(PyPy, link_name = "PyPyErr_NewException")]
     pub fn PyErr_NewException(
         name: *const c_char,
@@ -293,13 +293,13 @@ extern "C" {
         dict: *mut PyObject,
     ) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyErr_WriteUnraisable")]
-    pub fn PyErr_WriteUnraisable(arg1: *mut PyObject) -> ();
+    pub fn PyErr_WriteUnraisable(arg1: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyErr_CheckSignals")]
     pub fn PyErr_CheckSignals() -> c_int;
     #[cfg_attr(PyPy, link_name = "PyPyErr_SetInterrupt")]
-    pub fn PyErr_SetInterrupt() -> ();
-    pub fn PyErr_SyntaxLocation(filename: *const c_char, lineno: c_int) -> ();
-    pub fn PyErr_SyntaxLocationEx(filename: *const c_char, lineno: c_int, col_offset: c_int) -> ();
+    pub fn PyErr_SetInterrupt();
+    pub fn PyErr_SyntaxLocation(filename: *const c_char, lineno: c_int);
+    pub fn PyErr_SyntaxLocationEx(filename: *const c_char, lineno: c_int, col_offset: c_int);
     pub fn PyErr_ProgramText(filename: *const c_char, lineno: c_int) -> *mut PyObject;
     #[cfg(not(PyPy))]
     pub fn PyUnicodeDecodeError_Create(
diff --git a/src/ffi/pymem.rs b/src/ffi/pymem.rs
index 5bb517f2..58e3e897 100644
--- a/src/ffi/pymem.rs
+++ b/src/ffi/pymem.rs
@@ -11,7 +11,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyMem_RawRealloc")]
     pub fn PyMem_RawRealloc(ptr: *mut c_void, new_size: size_t) -> *mut c_void;
     #[cfg_attr(PyPy, link_name = "PyPyMem_RawFree")]
-    pub fn PyMem_RawFree(ptr: *mut c_void) -> ();
+    pub fn PyMem_RawFree(ptr: *mut c_void);
 }
 
 #[cfg_attr(windows, link(name = "pythonXY"))]
@@ -23,7 +23,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyMem_Realloc")]
     pub fn PyMem_Realloc(ptr: *mut c_void, new_size: size_t) -> *mut c_void;
     #[cfg_attr(PyPy, link_name = "PyPyMem_Free")]
-    pub fn PyMem_Free(ptr: *mut c_void) -> ();
+    pub fn PyMem_Free(ptr: *mut c_void);
 }
 
 #[cfg(not(Py_LIMITED_API))]
@@ -45,15 +45,13 @@ pub struct PyMemAllocatorEx {
         Option<extern "C" fn(ctx: *mut c_void, nelem: size_t, elsize: size_t) -> *mut c_void>,
     pub realloc:
         Option<extern "C" fn(ctx: *mut c_void, ptr: *mut c_void, new_size: size_t) -> *mut c_void>,
-    pub free: Option<extern "C" fn(ctx: *mut c_void, ptr: *mut c_void) -> ()>,
+    pub free: Option<extern "C" fn(ctx: *mut c_void, ptr: *mut c_void)>,
 }
 
 #[cfg(not(Py_LIMITED_API))]
 #[cfg_attr(windows, link(name = "pythonXY"))]
 extern "C" {
-    pub fn PyMem_GetAllocator(domain: PyMemAllocatorDomain, allocator: *mut PyMemAllocatorEx)
-        -> ();
-    pub fn PyMem_SetAllocator(domain: PyMemAllocatorDomain, allocator: *mut PyMemAllocatorEx)
-        -> ();
-    pub fn PyMem_SetupDebugHooks() -> ();
+    pub fn PyMem_GetAllocator(domain: PyMemAllocatorDomain, allocator: *mut PyMemAllocatorEx);
+    pub fn PyMem_SetAllocator(domain: PyMemAllocatorDomain, allocator: *mut PyMemAllocatorEx);
+    pub fn PyMem_SetupDebugHooks();
 }
diff --git a/src/ffi/pystate.rs b/src/ffi/pystate.rs
index 92c43422..2fbd2ea3 100644
--- a/src/ffi/pystate.rs
+++ b/src/ffi/pystate.rs
@@ -25,8 +25,8 @@ pub struct PyThreadState {
 #[cfg_attr(windows, link(name = "pythonXY"))]
 extern "C" {
     pub fn PyInterpreterState_New() -> *mut PyInterpreterState;
-    pub fn PyInterpreterState_Clear(arg1: *mut PyInterpreterState) -> ();
-    pub fn PyInterpreterState_Delete(arg1: *mut PyInterpreterState) -> ();
+    pub fn PyInterpreterState_Clear(arg1: *mut PyInterpreterState);
+    pub fn PyInterpreterState_Delete(arg1: *mut PyInterpreterState);
     //fn _PyState_AddModule(arg1: *mut PyObject,
     //                      arg2: *mut PyModuleDef) -> c_int;
     pub fn PyState_FindModule(arg1: *mut PyModuleDef) -> *mut PyObject;
@@ -34,14 +34,14 @@ extern "C" {
     pub fn PyThreadState_New(arg1: *mut PyInterpreterState) -> *mut PyThreadState;
     //fn _PyThreadState_Prealloc(arg1: *mut PyInterpreterState)
     // -> *mut PyThreadState;
-    //fn _PyThreadState_Init(arg1: *mut PyThreadState) -> ();
+    //fn _PyThreadState_Init(arg1: *mut PyThreadState);
     #[cfg_attr(PyPy, link_name = "PyPyThreadState_Clear")]
-    pub fn PyThreadState_Clear(arg1: *mut PyThreadState) -> ();
+    pub fn PyThreadState_Clear(arg1: *mut PyThreadState);
     #[cfg_attr(PyPy, link_name = "PyPyThreadState_Delete")]
-    pub fn PyThreadState_Delete(arg1: *mut PyThreadState) -> ();
+    pub fn PyThreadState_Delete(arg1: *mut PyThreadState);
     #[cfg(py_sys_config = "WITH_THREAD")]
     #[cfg_attr(PyPy, link_name = "PyPyThreadState_DeleteCurrent")]
-    pub fn PyThreadState_DeleteCurrent() -> ();
+    pub fn PyThreadState_DeleteCurrent();
     #[cfg_attr(PyPy, link_name = "PyPyThreadState_Get")]
     pub fn PyThreadState_Get() -> *mut PyThreadState;
     #[cfg_attr(PyPy, link_name = "PyPyThreadState_Swap")]
@@ -63,7 +63,7 @@ extern "C" {
     #[cfg_attr(PyPy, link_name = "PyPyGILState_Ensure")]
     pub fn PyGILState_Ensure() -> PyGILState_STATE;
     #[cfg_attr(PyPy, link_name = "PyPyGILState_Release")]
-    pub fn PyGILState_Release(arg1: PyGILState_STATE) -> ();
+    pub fn PyGILState_Release(arg1: PyGILState_STATE);
     pub fn PyGILState_GetThisThreadState() -> *mut PyThreadState;
 }
 
diff --git a/src/ffi/pythonrun.rs b/src/ffi/pythonrun.rs
index 03041b53..2252ac10 100644
--- a/src/ffi/pythonrun.rs
+++ b/src/ffi/pythonrun.rs
@@ -11,18 +11,18 @@ use std::ptr;
 #[cfg_attr(windows, link(name = "pythonXY"))]
 extern "C" {
     // TODO: these moved to pylifecycle.h
-    pub fn Py_SetProgramName(arg1: *mut wchar_t) -> ();
+    pub fn Py_SetProgramName(arg1: *mut wchar_t);
     #[cfg_attr(PyPy, link_name = "PyPy_GetProgramName")]
     pub fn Py_GetProgramName() -> *mut wchar_t;
-    pub fn Py_SetPythonHome(arg1: *mut wchar_t) -> ();
+    pub fn Py_SetPythonHome(arg1: *mut wchar_t);
     pub fn Py_GetPythonHome() -> *mut wchar_t;
-    pub fn Py_Initialize() -> ();
-    pub fn Py_InitializeEx(arg1: c_int) -> ();
-    pub fn Py_Finalize() -> ();
+    pub fn Py_Initialize();
+    pub fn Py_InitializeEx(arg1: c_int);
+    pub fn Py_Finalize();
     #[cfg_attr(PyPy, link_name = "PyPy_IsInitialized")]
     pub fn Py_IsInitialized() -> c_int;
     pub fn Py_NewInterpreter() -> *mut PyThreadState;
-    pub fn Py_EndInterpreter(arg1: *mut PyThreadState) -> ();
+    pub fn Py_EndInterpreter(arg1: *mut PyThreadState);
 }
 
 #[repr(C)]
@@ -220,22 +220,22 @@ extern "C" {
     ) -> *mut symtable;
 
     #[cfg_attr(PyPy, link_name = "PyPyErr_Print")]
-    pub fn PyErr_Print() -> ();
+    pub fn PyErr_Print();
     #[cfg_attr(PyPy, link_name = "PyPyErr_PrintEx")]
-    pub fn PyErr_PrintEx(arg1: c_int) -> ();
+    pub fn PyErr_PrintEx(arg1: c_int);
     #[cfg_attr(PyPy, link_name = "PyPyErr_Display")]
-    pub fn PyErr_Display(arg1: *mut PyObject, arg2: *mut PyObject, arg3: *mut PyObject) -> ();
+    pub fn PyErr_Display(arg1: *mut PyObject, arg2: *mut PyObject, arg3: *mut PyObject);
 
     // TODO: these moved to pylifecycle.h
     #[cfg_attr(PyPy, link_name = "PyPy_AtExit")]
-    pub fn Py_AtExit(func: Option<extern "C" fn() -> ()>) -> c_int;
-    pub fn Py_Exit(arg1: c_int) -> ();
+    pub fn Py_AtExit(func: Option<extern "C" fn()>) -> c_int;
+    pub fn Py_Exit(arg1: c_int);
     pub fn Py_Main(argc: c_int, argv: *mut *mut wchar_t) -> c_int;
     pub fn Py_GetProgramFullPath() -> *mut wchar_t;
     pub fn Py_GetPrefix() -> *mut wchar_t;
     pub fn Py_GetExecPrefix() -> *mut wchar_t;
     pub fn Py_GetPath() -> *mut wchar_t;
-    pub fn Py_SetPath(arg1: *const wchar_t) -> ();
+    pub fn Py_SetPath(arg1: *const wchar_t);
     #[cfg_attr(PyPy, link_name = "PyPy_GetVersion")]
     pub fn Py_GetVersion() -> *const c_char;
     pub fn Py_GetPlatform() -> *const c_char;
diff --git a/src/ffi/structseq.rs b/src/ffi/structseq.rs
index 96857a4c..1829fd51 100644
--- a/src/ffi/structseq.rs
+++ b/src/ffi/structseq.rs
@@ -22,10 +22,6 @@ pub struct PyStructSequence_Desc {
 extern "C" {
     pub fn PyStructSequence_NewType(desc: *mut PyStructSequence_Desc) -> *mut PyTypeObject;
     pub fn PyStructSequence_New(_type: *mut PyTypeObject) -> *mut PyObject;
-    pub fn PyStructSequence_SetItem(
-        arg1: *mut PyObject,
-        arg2: Py_ssize_t,
-        arg3: *mut PyObject,
-    ) -> ();
+    pub fn PyStructSequence_SetItem(arg1: *mut PyObject, arg2: Py_ssize_t, arg3: *mut PyObject);
     pub fn PyStructSequence_GetItem(arg1: *mut PyObject, arg2: Py_ssize_t) -> *mut PyObject;
 }
diff --git a/src/ffi/sysmodule.rs b/src/ffi/sysmodule.rs
index 3af451c2..a96cfec6 100644
--- a/src/ffi/sysmodule.rs
+++ b/src/ffi/sysmodule.rs
@@ -10,19 +10,19 @@ extern "C" {
     pub fn PySys_GetObject(arg1: *const c_char) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPySys_SetObject")]
     pub fn PySys_SetObject(arg1: *const c_char, arg2: *mut PyObject) -> c_int;
-    pub fn PySys_SetArgv(arg1: c_int, arg2: *mut *mut wchar_t) -> ();
-    pub fn PySys_SetArgvEx(arg1: c_int, arg2: *mut *mut wchar_t, arg3: c_int) -> ();
-    pub fn PySys_SetPath(arg1: *const wchar_t) -> ();
+    pub fn PySys_SetArgv(arg1: c_int, arg2: *mut *mut wchar_t);
+    pub fn PySys_SetArgvEx(arg1: c_int, arg2: *mut *mut wchar_t, arg3: c_int);
+    pub fn PySys_SetPath(arg1: *const wchar_t);
     #[cfg_attr(PyPy, link_name = "PyPySys_WriteStdout")]
-    pub fn PySys_WriteStdout(format: *const c_char, ...) -> ();
+    pub fn PySys_WriteStdout(format: *const c_char, ...);
     #[cfg_attr(PyPy, link_name = "PyPySys_WriteStderr")]
-    pub fn PySys_WriteStderr(format: *const c_char, ...) -> ();
-    pub fn PySys_FormatStdout(format: *const c_char, ...) -> ();
-    pub fn PySys_FormatStderr(format: *const c_char, ...) -> ();
-    pub fn PySys_ResetWarnOptions() -> ();
-    pub fn PySys_AddWarnOption(arg1: *const wchar_t) -> ();
-    pub fn PySys_AddWarnOptionUnicode(arg1: *mut PyObject) -> ();
+    pub fn PySys_WriteStderr(format: *const c_char, ...);
+    pub fn PySys_FormatStdout(format: *const c_char, ...);
+    pub fn PySys_FormatStderr(format: *const c_char, ...);
+    pub fn PySys_ResetWarnOptions();
+    pub fn PySys_AddWarnOption(arg1: *const wchar_t);
+    pub fn PySys_AddWarnOptionUnicode(arg1: *mut PyObject);
     pub fn PySys_HasWarnOptions() -> c_int;
-    pub fn PySys_AddXOption(arg1: *const wchar_t) -> ();
+    pub fn PySys_AddXOption(arg1: *const wchar_t);
     pub fn PySys_GetXOptions() -> *mut PyObject;
 }
diff --git a/src/ffi/unicodeobject.rs b/src/ffi/unicodeobject.rs
index 2b8e5dec..8f178d11 100644
--- a/src/ffi/unicodeobject.rs
+++ b/src/ffi/unicodeobject.rs
@@ -116,8 +116,8 @@ extern "C" {
     //                             vargs: va_list) -> *mut PyObject;
     pub fn PyUnicode_FromFormat(format: *const c_char, ...) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyUnicode_InternInPlace")]
-    pub fn PyUnicode_InternInPlace(arg1: *mut *mut PyObject) -> ();
-    pub fn PyUnicode_InternImmortal(arg1: *mut *mut PyObject) -> ();
+    pub fn PyUnicode_InternInPlace(arg1: *mut *mut PyObject);
+    pub fn PyUnicode_InternImmortal(arg1: *mut *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyUnicode_InternFromString")]
     pub fn PyUnicode_InternFromString(u: *const c_char) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyUnicode_FromWideChar")]
@@ -389,8 +389,8 @@ extern "C" {
     pub fn PyUnicode_EncodeFSDefault(unicode: *mut PyObject) -> *mut PyObject;
     #[cfg_attr(PyPy, link_name = "PyPyUnicode_Concat")]
     pub fn PyUnicode_Concat(left: *mut PyObject, right: *mut PyObject) -> *mut PyObject;
-    pub fn PyUnicode_Append(pleft: *mut *mut PyObject, right: *mut PyObject) -> ();
-    pub fn PyUnicode_AppendAndDel(pleft: *mut *mut PyObject, right: *mut PyObject) -> ();
+    pub fn PyUnicode_Append(pleft: *mut *mut PyObject, right: *mut PyObject);
+    pub fn PyUnicode_AppendAndDel(pleft: *mut *mut PyObject, right: *mut PyObject);
     #[cfg_attr(PyPy, link_name = "PyPyUnicode_Split")]
     pub fn PyUnicode_Split(
         s: *mut PyObject,
diff --git a/src/gil.rs b/src/gil.rs
index a2ef2154..642261f2 100644
--- a/src/gil.rs
+++ b/src/gil.rs
@@ -108,7 +108,6 @@ impl Drop for GILGuard {
         unsafe {
             let pool: &'static mut ReleasePool = &mut *POOL;
             pool.drain(self.python(), self.owned, self.borrowed);
-
             ffi::PyGILState_Release(self.gstate);
         }
     }
diff --git a/src/internal_tricks.rs b/src/internal_tricks.rs
index 0dae9fb8..a5388165 100644
--- a/src/internal_tricks.rs
+++ b/src/internal_tricks.rs
@@ -12,7 +12,7 @@ macro_rules! private_decl {
         /// This trait is private to implement; this method exists to make it
         /// impossible to implement outside the crate.
         fn __private__(&self) -> crate::internal_tricks::PrivateMarker;
-    }
+    };
 }
 
 macro_rules! private_impl {
@@ -21,7 +21,7 @@ macro_rules! private_impl {
         fn __private__(&self) -> crate::internal_tricks::PrivateMarker {
             crate::internal_tricks::PrivateMarker
         }
-    }
+    };
 }
 
 macro_rules! pyo3_exception {
diff --git a/src/lib.rs b/src/lib.rs
index 033e9c93..3b501a22 100755
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -133,6 +133,7 @@
 //! }
 //! ```
 
+pub use crate::callback::run_callback;
 pub use crate::class::*;
 pub use crate::conversion::{
     AsPyPointer, FromPy, FromPyObject, FromPyPointer, IntoPy, IntoPyPointer, PyTryFrom, PyTryInto,
diff --git a/src/python.rs b/src/python.rs
index 0d9b479c..6c4f4d89 100644
--- a/src/python.rs
+++ b/src/python.rs
@@ -9,8 +9,7 @@ use crate::instance::AsPyRef;
 use crate::object::PyObject;
 use crate::type_object::{PyDowncastImpl, PyTypeInfo, PyTypeObject};
 use crate::types::{PyAny, PyDict, PyModule, PyType};
-use crate::AsPyPointer;
-use crate::{FromPyPointer, IntoPyPointer, PyTryFrom};
+use crate::{AsPyPointer, FromPyPointer, IntoPyPointer, PyTryFrom};
 use std::ffi::CString;
 use std::marker::PhantomData;
 use std::os::raw::c_int;
diff --git a/src/types/set.rs b/src/types/set.rs
index 39a1afed..6b7aa248 100644
--- a/src/types/set.rs
+++ b/src/types/set.rs
@@ -4,8 +4,8 @@
 use crate::err::{self, PyErr, PyResult};
 use crate::internal_tricks::Unsendable;
 use crate::{
-    ffi, AsPyPointer, FromPyObject, PyAny, PyNativeType, PyObject, PyTryFrom, Python,
-    ToBorrowedObject, ToPyObject,
+    ffi, AsPyPointer, FromPyObject, PyAny, PyNativeType, PyObject, Python, ToBorrowedObject,
+    ToPyObject,
 };
 use std::cmp;
 use std::collections::{BTreeSet, HashSet};
@@ -189,13 +189,9 @@ where
     K: FromPyObject<'source> + cmp::Eq + hash::Hash,
     S: hash::BuildHasher + Default,
 {
-    fn extract(ob: &'source PyAny) -> Result<Self, PyErr> {
-        let set = <PySet as PyTryFrom>::try_from(ob)?;
-        let mut ret = HashSet::default();
-        for k in set.iter() {
-            ret.insert(K::extract(k)?);
-        }
-        Ok(ret)
+    fn extract(ob: &'source PyAny) -> PyResult<Self> {
+        let set: &PySet = ob.downcast()?;
+        set.iter().map(K::extract).collect()
     }
 }
 
@@ -203,13 +199,9 @@ impl<'source, K> FromPyObject<'source> for BTreeSet<K>
 where
     K: FromPyObject<'source> + cmp::Ord,
 {
-    fn extract(ob: &'source PyAny) -> Result<Self, PyErr> {
-        let set = <PySet as PyTryFrom>::try_from(ob)?;
-        let mut ret = BTreeSet::default();
-        for k in set.iter() {
-            ret.insert(K::extract(k)?);
-        }
-        Ok(ret)
+    fn extract(ob: &'source PyAny) -> PyResult<Self> {
+        let set: &PySet = ob.downcast()?;
+        set.iter().map(K::extract).collect()
     }
 }
 
@@ -279,9 +271,9 @@ impl<'a> std::iter::IntoIterator for &'a PyFrozenSet {
 #[cfg(test)]
 mod test {
     use super::{PyFrozenSet, PySet};
-    use crate::instance::AsPyRef;
-    use crate::{ObjectProtocol, PyTryFrom, Python, ToPyObject};
-    use std::collections::HashSet;
+    use crate::{AsPyRef, ObjectProtocol, PyTryFrom, Python, ToPyObject};
+    use std::collections::{BTreeSet, HashSet};
+    use std::iter::FromIterator;
 
     #[test]
     fn test_set_new() {
@@ -433,4 +425,30 @@ mod test {
             assert_eq!(1i32, el.extract::<i32>().unwrap());
         }
     }
+
+    #[test]
+    fn test_extract_hashset() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+
+        let set = PySet::new(py, &[1, 2, 3, 4, 5]).unwrap();
+        let hash_set: HashSet<usize> = set.extract().unwrap();
+        assert_eq!(
+            hash_set,
+            HashSet::from_iter([1, 2, 3, 4, 5].iter().copied())
+        );
+    }
+
+    #[test]
+    fn test_extract_btreeset() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+
+        let set = PySet::new(py, &[1, 2, 3, 4, 5]).unwrap();
+        let hash_set: BTreeSet<usize> = set.extract().unwrap();
+        assert_eq!(
+            hash_set,
+            BTreeSet::from_iter([1, 2, 3, 4, 5].iter().copied())
+        );
+    }
 }
diff --git a/tests/test_arithmetics.rs b/tests/test_arithmetics.rs
index 914585cd..8b1260dc 100644
--- a/tests/test_arithmetics.rs
+++ b/tests/test_arithmetics.rs
@@ -8,24 +8,39 @@ use pyo3::py_run;
 mod common;
 
 #[pyclass]
-struct UnaryArithmetic {}
+struct UnaryArithmetic {
+    inner: f64,
+}
+
+impl UnaryArithmetic {
+    fn new(value: f64) -> Self {
+        UnaryArithmetic { inner: value }
+    }
+}
+
+#[pyproto]
+impl PyObjectProtocol for UnaryArithmetic {
+    fn __repr__(&self) -> PyResult<String> {
+        Ok(format!("UA({})", self.inner))
+    }
+}
 
 #[pyproto]
 impl PyNumberProtocol for UnaryArithmetic {
-    fn __neg__(&self) -> PyResult<&'static str> {
-        Ok("neg")
+    fn __neg__(&self) -> PyResult<Self> {
+        Ok(Self::new(-self.inner))
     }
 
-    fn __pos__(&self) -> PyResult<&'static str> {
-        Ok("pos")
+    fn __pos__(&self) -> PyResult<Self> {
+        Ok(Self::new(self.inner))
     }
 
-    fn __abs__(&self) -> PyResult<&'static str> {
-        Ok("abs")
+    fn __abs__(&self) -> PyResult<Self> {
+        Ok(Self::new(self.inner.abs()))
     }
 
-    fn __invert__(&self) -> PyResult<&'static str> {
-        Ok("invert")
+    fn __round__(&self, _ndigits: Option<u32>) -> PyResult<Self> {
+        Ok(Self::new(self.inner.round()))
     }
 }
 
@@ -34,11 +49,12 @@ fn unary_arithmetic() {
     let gil = Python::acquire_gil();
     let py = gil.python();
 
-    let c = Py::new(py, UnaryArithmetic {}).unwrap();
-    py_run!(py, c, "assert -c == 'neg'");
-    py_run!(py, c, "assert +c == 'pos'");
-    py_run!(py, c, "assert abs(c) == 'abs'");
-    py_run!(py, c, "assert ~c == 'invert'");
+    let c = PyCell::new(py, UnaryArithmetic::new(2.7)).unwrap();
+    py_run!(py, c, "assert repr(-c) == 'UA(-2.7)'");
+    py_run!(py, c, "assert repr(+c) == 'UA(2.7)'");
+    py_run!(py, c, "assert repr(abs(c)) == 'UA(2.7)'");
+    py_run!(py, c, "assert repr(round(c)) == 'UA(3)'");
+    py_run!(py, c, "assert repr(round(c, 1)) == 'UA(3)'");
 }
 
 #[pyclass]
@@ -104,15 +120,19 @@ impl PyNumberProtocol for InPlaceOperations {
         self.value |= other;
         Ok(())
     }
+
+    fn __ipow__(&mut self, other: u32) -> PyResult<()> {
+        self.value = self.value.pow(other);
+        Ok(())
+    }
 }
 
 #[test]
 fn inplace_operations() {
     let gil = Python::acquire_gil();
     let py = gil.python();
-
     let init = |value, code| {
-        let c = Py::new(py, InPlaceOperations { value }).unwrap();
+        let c = PyCell::new(py, InPlaceOperations { value }).unwrap();
         py_run!(py, c, code);
     };
 
@@ -124,6 +144,11 @@ fn inplace_operations() {
     init(12, "d = c; c &= 10; assert repr(c) == repr(d) == 'IPO(8)'");
     init(12, "d = c; c |= 3; assert repr(c) == repr(d) == 'IPO(15)'");
     init(12, "d = c; c ^= 5; assert repr(c) == repr(d) == 'IPO(9)'");
+    init(3, "d = c; c **= 4; assert repr(c) == repr(d) == 'IPO(81)'");
+    init(
+        3,
+        "d = c; c.__ipow__(4); assert repr(c) == repr(d) == 'IPO(81)'",
+    );
 }
 
 #[pyproto]
@@ -159,6 +184,10 @@ impl PyNumberProtocol for BinaryArithmetic {
     fn __or__(lhs: &PyAny, rhs: &PyAny) -> PyResult<String> {
         Ok(format!("{:?} | {:?}", lhs, rhs))
     }
+
+    fn __pow__(lhs: &PyAny, rhs: &PyAny, mod_: Option<u32>) -> PyResult<String> {
+        Ok(format!("{:?} ** {:?} (mod: {:?})", lhs, rhs, mod_))
+    }
 }
 
 #[test]
@@ -166,8 +195,9 @@ fn binary_arithmetic() {
     let gil = Python::acquire_gil();
     let py = gil.python();
 
-    let c = Py::new(py, BinaryArithmetic {}).unwrap();
+    let c = PyCell::new(py, BinaryArithmetic {}).unwrap();
     py_run!(py, c, "assert c + c == 'BA + BA'");
+    py_run!(py, c, "assert c.__add__(c) == 'BA + BA'");
     py_run!(py, c, "assert c + 1 == 'BA + 1'");
     py_run!(py, c, "assert 1 + c == '1 + BA'");
     py_run!(py, c, "assert c - 1 == 'BA - 1'");
@@ -185,6 +215,10 @@ fn binary_arithmetic() {
     py_run!(py, c, "assert 1 ^ c == '1 ^ BA'");
     py_run!(py, c, "assert c | 1 == 'BA | 1'");
     py_run!(py, c, "assert 1 | c == '1 | BA'");
+    py_run!(py, c, "assert c ** 1 == 'BA ** 1 (mod: None)'");
+    py_run!(py, c, "assert 1 ** c == '1 ** BA (mod: None)'");
+
+    py_run!(py, c, "assert pow(c, 1, 100) == 'BA ** 1 (mod: Some(100))'");
 }
 
 #[pyclass]
@@ -195,6 +229,38 @@ impl PyNumberProtocol for RhsArithmetic {
     fn __radd__(&self, other: &PyAny) -> PyResult<String> {
         Ok(format!("{:?} + RA", other))
     }
+
+    fn __rsub__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} - RA", other))
+    }
+
+    fn __rmul__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} * RA", other))
+    }
+
+    fn __rlshift__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} << RA", other))
+    }
+
+    fn __rrshift__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} >> RA", other))
+    }
+
+    fn __rand__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} & RA", other))
+    }
+
+    fn __rxor__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} ^ RA", other))
+    }
+
+    fn __ror__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} | RA", other))
+    }
+
+    fn __rpow__(&self, other: &PyAny, _mod: Option<&'p PyAny>) -> PyResult<String> {
+        Ok(format!("{:?} ** RA", other))
+    }
 }
 
 #[test]
@@ -202,11 +268,78 @@ fn rhs_arithmetic() {
     let gil = Python::acquire_gil();
     let py = gil.python();
 
-    let c = Py::new(py, RhsArithmetic {}).unwrap();
+    let c = PyCell::new(py, RhsArithmetic {}).unwrap();
     py_run!(py, c, "assert c.__radd__(1) == '1 + RA'");
-    // TODO: commented out for now until reflected arithemtics gets fixed.
-    // see discussion here: https://github.com/PyO3/pyo3/pull/550
-    // py_run!(py, c, "assert 1 + c == '1 + RA'");
+    py_run!(py, c, "assert 1 + c == '1 + RA'");
+    py_run!(py, c, "assert c.__rsub__(1) == '1 - RA'");
+    py_run!(py, c, "assert 1 - c == '1 - RA'");
+    py_run!(py, c, "assert c.__rmul__(1) == '1 * RA'");
+    py_run!(py, c, "assert 1 * c == '1 * RA'");
+    py_run!(py, c, "assert c.__rlshift__(1) == '1 << RA'");
+    py_run!(py, c, "assert 1 << c == '1 << RA'");
+    py_run!(py, c, "assert c.__rrshift__(1) == '1 >> RA'");
+    py_run!(py, c, "assert 1 >> c == '1 >> RA'");
+    py_run!(py, c, "assert c.__rand__(1) == '1 & RA'");
+    py_run!(py, c, "assert 1 & c == '1 & RA'");
+    py_run!(py, c, "assert c.__rxor__(1) == '1 ^ RA'");
+    py_run!(py, c, "assert 1 ^ c == '1 ^ RA'");
+    py_run!(py, c, "assert c.__ror__(1) == '1 | RA'");
+    py_run!(py, c, "assert 1 | c == '1 | RA'");
+    py_run!(py, c, "assert c.__rpow__(1) == '1 ** RA'");
+    py_run!(py, c, "assert 1 ** c == '1 ** RA'");
+}
+
+#[pyclass]
+struct LhsAndRhsArithmetic {}
+
+#[pyproto]
+impl PyNumberProtocol for LhsAndRhsArithmetic {
+    fn __radd__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} + RA", other))
+    }
+
+    fn __rsub__(&self, other: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} - RA", other))
+    }
+
+    fn __rpow__(&self, other: &PyAny, _mod: Option<&'p PyAny>) -> PyResult<String> {
+        Ok(format!("{:?} ** RA", other))
+    }
+
+    fn __add__(lhs: &PyAny, rhs: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} + {:?}", lhs, rhs))
+    }
+
+    fn __sub__(lhs: &PyAny, rhs: &PyAny) -> PyResult<String> {
+        Ok(format!("{:?} - {:?}", lhs, rhs))
+    }
+
+    fn __pow__(lhs: &PyAny, rhs: &PyAny, _mod: Option<u32>) -> PyResult<String> {
+        Ok(format!("{:?} ** {:?}", lhs, rhs))
+    }
+}
+
+#[pyproto]
+impl PyObjectProtocol for LhsAndRhsArithmetic {
+    fn __repr__(&self) -> PyResult<&'static str> {
+        Ok("BA")
+    }
+}
+
+#[test]
+fn lhs_override_rhs() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+
+    let c = PyCell::new(py, LhsAndRhsArithmetic {}).unwrap();
+    // Not overrided
+    py_run!(py, c, "assert c.__radd__(1) == '1 + RA'");
+    py_run!(py, c, "assert c.__rsub__(1) == '1 - RA'");
+    py_run!(py, c, "assert c.__rpow__(1) == '1 ** RA'");
+    // Overrided
+    py_run!(py, c, "assert 1 + c == '1 + BA'");
+    py_run!(py, c, "assert 1 - c == '1 - BA'");
+    py_run!(py, c, "assert 1 ** c == '1 ** BA'");
 }
 
 #[pyclass]
@@ -254,7 +387,7 @@ fn rich_comparisons() {
     let gil = Python::acquire_gil();
     let py = gil.python();
 
-    let c = Py::new(py, RichComparisons {}).unwrap();
+    let c = PyCell::new(py, RichComparisons {}).unwrap();
     py_run!(py, c, "assert (c < c) == 'RC < RC'");
     py_run!(py, c, "assert (c < 1) == 'RC < 1'");
     py_run!(py, c, "assert (1 < c) == 'RC > 1'");
@@ -280,7 +413,7 @@ fn rich_comparisons_python_3_type_error() {
     let gil = Python::acquire_gil();
     let py = gil.python();
 
-    let c2 = Py::new(py, RichComparisons2 {}).unwrap();
+    let c2 = PyCell::new(py, RichComparisons2 {}).unwrap();
     py_expect_exception!(py, c2, "c2 < c2", TypeError);
     py_expect_exception!(py, c2, "c2 < 1", TypeError);
     py_expect_exception!(py, c2, "1 < c2", TypeError);
diff --git a/tests/test_gc.rs b/tests/test_gc.rs
index 829bb881..88e440fd 100644
--- a/tests/test_gc.rs
+++ b/tests/test_gc.rs
@@ -2,6 +2,7 @@ use pyo3::class::PyGCProtocol;
 use pyo3::class::PyTraverseError;
 use pyo3::class::PyVisit;
 use pyo3::prelude::*;
+use pyo3::type_object::PyTypeObject;
 use pyo3::{py_run, AsPyPointer, PyCell, PyTryInto};
 use std::cell::RefCell;
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -245,3 +246,63 @@ fn inheritance_with_new_methods_with_drop() {
     assert!(drop_called1.load(Ordering::Relaxed));
     assert!(drop_called2.load(Ordering::Relaxed));
 }
+
+#[pyclass(gc)]
+struct TraversableClass {
+    traversed: AtomicBool,
+}
+
+impl TraversableClass {
+    fn new() -> Self {
+        Self {
+            traversed: AtomicBool::new(false),
+        }
+    }
+}
+
+#[pyproto]
+impl PyGCProtocol for TraversableClass {
+    fn __clear__(&mut self) {}
+    fn __traverse__(&self, _visit: PyVisit) -> Result<(), PyTraverseError> {
+        self.traversed.store(true, Ordering::Relaxed);
+        Ok(())
+    }
+}
+
+#[test]
+fn gc_during_borrow() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+
+    unsafe {
+        // declare a dummy visitor function
+        extern "C" fn novisit(
+            _object: *mut pyo3::ffi::PyObject,
+            _arg: *mut core::ffi::c_void,
+        ) -> std::os::raw::c_int {
+            0
+        }
+
+        // get the traverse function
+        let ty = TraversableClass::type_object().as_ref(py).as_type_ptr();
+        let traverse = (*ty).tp_traverse.unwrap();
+
+        // create an object and check that traversing it works normally
+        // when it's not borrowed
+        let cell = PyCell::new(py, TraversableClass::new()).unwrap();
+        let obj = cell.to_object(py);
+        assert!(!cell.borrow().traversed.load(Ordering::Relaxed));
+        traverse(obj.as_ptr(), novisit, std::ptr::null_mut());
+        assert!(cell.borrow().traversed.load(Ordering::Relaxed));
+
+        // create an object and check that it is not traversed if the GC
+        // is invoked while it is already borrowed mutably
+        let cell2 = PyCell::new(py, TraversableClass::new()).unwrap();
+        let obj2 = cell2.to_object(py);
+        let guard = cell2.borrow_mut();
+        assert!(!guard.traversed.load(Ordering::Relaxed));
+        traverse(obj2.as_ptr(), novisit, std::ptr::null_mut());
+        assert!(!guard.traversed.load(Ordering::Relaxed));
+        drop(guard);
+    }
+}