Relax return type requirements

Allows returning essentially arbitrary types by wrapping them into a PyResult. This is done with a conversion trait that specializes for PyResult.

pyo3-derive-backend/src/args.rs

@@ -1,7 +1,7 @@
 // Copyright (c) 2017-present PyO3 Project and Contributors
 use syn;
 
-#[derive(Debug, PartialEq)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum Argument {
     VarArgsSeparator,
     VarArgs(String),

pyo3-derive-backend/src/method.rs

@@ -1,13 +1,12 @@
 // Copyright (c) 2017-present PyO3 Project and Contributors
 
-use syn;
-use quote::{Tokens, Ident};
-
 use args::{Argument, parse_arguments};
+use quote::{Ident, Tokens};
+use syn;
 use utils::for_err_msg;
 
 
-#[derive(Clone, Debug)]
+#[derive(Clone, PartialEq, Debug)]
 pub struct FnArg<'a> {
     pub name: &'a syn::Ident,
     pub mode: &'a syn::BindingMode,
@@ -29,6 +28,7 @@ pub enum FnType {
     FnStatic,
 }
 
+#[derive(Clone, PartialEq, Debug)]
 pub struct FnSpec<'a> {
     pub tp: FnType,
     pub attrs: Vec<Argument>,
@@ -36,8 +36,14 @@ pub struct FnSpec<'a> {
     pub output: syn::Ty,
 }
 
-impl<'a> FnSpec<'a> {
+pub fn get_return_info(output: &syn::FunctionRetTy) -> syn::Ty {
+    match output {
+        syn::FunctionRetTy::Default => syn::Ty::Tup(vec![]),
+        syn::FunctionRetTy::Ty(ref ty) => ty.clone()
+    }
+}
 
+impl<'a> FnSpec<'a> {
     /// Parser function signature and function attributes
     pub fn parse(name: &'a syn::Ident,
                  sig: &'a syn::MethodSig,
@@ -96,10 +102,7 @@ impl<'a> FnSpec<'a> {
             }
         }
 
-        let ty = match sig.decl.output {
+        let ty = get_return_info(&sig.decl.output);
-            syn::FunctionRetTy::Default => syn::Ty::Infer,
-            syn::FunctionRetTy::Ty(ref ty) => ty.clone()
-        };
 
         FnSpec {
             tp: fn_type,

pyo3-derive-backend/src/module.rs

@@ -227,22 +227,21 @@ fn wrap_fn(item: &mut syn::Item) -> Option<Box<syn::Block>> {
                         };
 
                         let opt = method::check_arg_ty_and_optional(&name, ty);
-                        arguments.push(method::FnArg {name: ident,
+                        arguments.push(method::FnArg {
+                            name: ident,
                             mode: mode,
                             ty: ty,
                             optional: opt,
                             py: py,
-                                                      reference: method::is_ref(&name, ty)});
+                            reference: method::is_ref(&name, ty),
+                        });
                     }
                     &syn::FnArg::Ignored(_) =>
                         panic!("ignored argument: {:?}", name),
                 }
             }
 
-            let ty = match decl.output {
+            let ty = method::get_return_info(&decl.output);
-                syn::FunctionRetTy::Default => syn::Ty::Infer,
-                syn::FunctionRetTy::Ty(ref ty) => ty.clone()
-            };
 
             let spec = method::FnSpec {
                 tp: method::FnType::Fn,
@@ -309,11 +308,11 @@ pub fn impl_wrap(name: &syn::Ident, spec: &method::FnSpec) -> Tokens {
         |item| if item.1.py {syn::Ident::from("_py")} else {
             syn::Ident::from(format!("arg{}", item.0))}).collect();
     let cb = quote! {{
-        #name(#(#names),*)
+        #name(#(#names),*).return_type_into_py_result()
     }};
 
     let body = py_method::impl_arg_params(spec, cb);
-    let output = &spec.output;
+    let body_to_result = py_method::body_to_result(&body, spec);
 
     quote! {
         #[allow(unused_variables, unused_imports)]
@@ -329,9 +328,7 @@ pub fn impl_wrap(name: &syn::Ident, spec: &method::FnSpec) -> Tokens {
             let _args = _py.from_borrowed_ptr::<_pyo3::PyTuple>(_args);
             let _kwargs = _pyo3::argparse::get_kwargs(_py, _kwargs);
 
-            let _result: #output = {
+            #body_to_result
-                #body
-            };
             _pyo3::callback::cb_convert(
                 _pyo3::callback::PyObjectCallbackConverter, _py, _result)
         }

pyo3-derive-backend/src/py_class.rs

@@ -337,7 +337,7 @@ fn impl_descriptors(cls: &syn::Ty, descriptors: Vec<(syn::Field, Vec<FnType>)>)
                             py: true,
                             reference: false
                         }],
-                        output: syn::parse::ty("PyResult<()>").expect("error parse PyResult<()>")
+                        output: syn::parse::ty("PyResult<()>").expect("error parse PyResult<()>"),
                     };
                     impl_py_setter_def(&name, doc, setter, &impl_wrap_setter(&Box::new(cls.clone()), &setter_name, &spec))
                 },

pyo3-derive-backend/src/py_method.rs

@@ -43,12 +43,23 @@ fn check_generic(name: &syn::Ident, sig: &syn::MethodSig) {
 }
 
 
+pub fn body_to_result(body: &Tokens, spec: &FnSpec) -> Tokens {
+    let output = &spec.output;
+    quote! {
+        use pyo3::ReturnTypeIntoPyResult;
+        let _result: PyResult<<#output as ReturnTypeIntoPyResult>::Inner> = {
+            #body
+        };
+    }
+}
+
 /// Generate function wrapper (PyCFunction, PyCFunctionWithKeywords)
 pub fn impl_wrap(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec, noargs: bool) -> Tokens {
-    let cb = impl_call(cls, name, &spec);
+    let body = impl_call(cls, name, &spec);
-    let output = &spec.output;
 
     if spec.args.is_empty() && noargs {
+        let body_to_result = body_to_result(&body, spec);
+
         quote! {
             unsafe extern "C" fn __wrap(
                 _slf: *mut _pyo3::ffi::PyObject) -> *mut _pyo3::ffi::PyObject
@@ -59,15 +70,14 @@ pub fn impl_wrap(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec, noargs: b
                 let _py = _pyo3::Python::assume_gil_acquired();
                 let _slf = _py.mut_from_borrowed_ptr::<#cls>(_slf);
 
-                let _result: #output = {
+                #body_to_result
-                    #cb
-                };
                 _pyo3::callback::cb_convert(
                     _pyo3::callback::PyObjectCallbackConverter, _py, _result)
             }
         }
     } else {
-        let body = impl_arg_params(&spec, cb);
+        let body = impl_arg_params(&spec, body);
+        let body_to_result = body_to_result(&body, spec);
 
         quote! {
             unsafe extern "C" fn __wrap(
@@ -84,9 +94,7 @@ pub fn impl_wrap(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec, noargs: b
                 let _args = _py.from_borrowed_ptr::<_pyo3::PyTuple>(_args);
                 let _kwargs = _pyo3::argparse::get_kwargs(_py, _kwargs);
 
-                let _result: #output = {
+                #body_to_result
-                    #body
-                };
                 _pyo3::callback::cb_convert(
                     _pyo3::callback::PyObjectCallbackConverter, _py, _result)
             }
@@ -128,11 +136,11 @@ pub fn impl_wrap_new(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) -> To
         |item| if item.1.py {syn::Ident::from("_py")} else {
             syn::Ident::from(format!("arg{}", item.0))}).collect();
     let cb = quote! {{
-        #cls::#name(&_obj, #(#names),*)
+        #cls::#name(&_obj, #(#names),*).return_type_into_py_result()
     }};
 
     let body = impl_arg_params(spec, cb);
-    let output = &spec.output;
+    let body_to_result = body_to_result(&body, spec);
 
     quote! {
         #[allow(unused_mut)]
@@ -152,9 +160,7 @@ pub fn impl_wrap_new(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) -> To
                     let _args = _py.from_borrowed_ptr::<_pyo3::PyTuple>(_args);
                     let _kwargs = _pyo3::argparse::get_kwargs(_py, _kwargs);
 
-                    let _result: #output = {
+                    #body_to_result
-                        #body
-                    };
 
                     match _result {
                         Ok(_) => _obj.into_ptr(),
@@ -176,7 +182,13 @@ pub fn impl_wrap_new(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) -> To
 /// Generate function wrapper for ffi::initproc
 fn impl_wrap_init(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) -> Tokens {
     let cb = impl_call(cls, name, &spec);
+    let output = &spec.output;
+    if quote! {#output} != quote! {PyResult<()>} || quote! {#output} != quote! {()}{
+        panic!("Constructor must return PyResult<()> or a ()");
+    }
+
     let body = impl_arg_params(&spec, cb);
+    let body_to_result = body_to_result(&body, spec);
 
     quote! {
         #[allow(unused_mut)]
@@ -192,9 +204,7 @@ fn impl_wrap_init(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) -> Token
             let _args = _py.from_borrowed_ptr::<_pyo3::PyTuple>(_args);
             let _kwargs = _pyo3::argparse::get_kwargs(_py, _kwargs);
 
-            let _result: PyResult<()> = {
+            #body_to_result
-                #body
-            };
             match _result {
                 Ok(_) => 0,
                 Err(e) => {
@@ -212,10 +222,11 @@ pub fn impl_wrap_class(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) ->
         |item| if item.1.py {syn::Ident::from("_py")} else {
             syn::Ident::from(format!("arg{}", item.0))}).collect();
     let cb = quote! {{
-        #cls::#name(&_cls, #(#names),*)
+        #cls::#name(&_cls, #(#names),*).return_type_into_py_result()
     }};
+
     let body = impl_arg_params(spec, cb);
-    let output = &spec.output;
+    let body_to_result = body_to_result(&body, spec);
 
     quote! {
         #[allow(unused_mut)]
@@ -231,9 +242,7 @@ pub fn impl_wrap_class(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) ->
             let _args = _py.from_borrowed_ptr::<_pyo3::PyTuple>(_args);
             let _kwargs = _pyo3::argparse::get_kwargs(_py, _kwargs);
 
-            let _result: #output = {
+            #body_to_result
-                #body
-            };
             _pyo3::callback::cb_convert(
                 _pyo3::callback::PyObjectCallbackConverter, _py, _result)
         }
@@ -246,11 +255,11 @@ pub fn impl_wrap_static(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) ->
         |item| if item.1.py {syn::Ident::from("_py")} else {
             syn::Ident::from(format!("arg{}", item.0))}).collect();
     let cb = quote! {{
-        #cls::#name(#(#names),*)
+        #cls::#name(#(#names),*).return_type_into_py_result()
     }};
 
     let body = impl_arg_params(spec, cb);
-    let output = &spec.output;
+    let body_to_result = body_to_result(&body, spec);
 
     quote! {
         #[allow(unused_mut)]
@@ -265,9 +274,7 @@ pub fn impl_wrap_static(cls: &Box<syn::Ty>, name: &syn::Ident, spec: &FnSpec) ->
             let _args = _py.from_borrowed_ptr::<_pyo3::PyTuple>(_args);
             let _kwargs = _pyo3::argparse::get_kwargs(_py, _kwargs);
 
-            let _result: #output = {
+            #body_to_result
-                #body
-            };
             _pyo3::callback::cb_convert(
                 _pyo3::callback::PyObjectCallbackConverter, _py, _result)
         }
@@ -344,7 +351,7 @@ fn impl_call(_cls: &Box<syn::Ty>, fname: &syn::Ident, spec: &FnSpec) -> Tokens {
         }
     ).collect();
     quote! {{
-        _slf.#fname(#(#names),*)
+        _slf.#fname(#(#names),*).return_type_into_py_result()
     }}
 }
 
@@ -393,6 +400,7 @@ pub fn impl_arg_params(spec: &FnSpec, body: Tokens) -> Tokens {
     let mut rargs = spec.args.clone();
     rargs.reverse();
     let mut body = body;
+
     for (idx, arg) in rargs.iter().enumerate() {
         body = impl_arg_param(&arg, &spec, &body, len-idx-1);
     }
@@ -424,7 +432,7 @@ pub fn impl_arg_params(spec: &FnSpec, body: Tokens) -> Tokens {
 
 fn impl_arg_param(arg: &FnArg, spec: &FnSpec, body: &Tokens, idx: usize) -> Tokens {
     if arg.py {
-        return body.clone()
+        return body.clone();
     }
     let ty = arg.ty;
     let name = arg.name;
@@ -444,19 +452,17 @@ fn impl_arg_param(arg: &FnArg, spec: &FnSpec, body: &Tokens, idx: usize) -> Toke
                 Err(e) => Err(e)
             }
         }
-    }
+    } else if spec.is_kwargs(&name) {
-    else if spec.is_kwargs(&name) {
         quote! {{
             let #arg_name = _kwargs;
             #body
         }}
-    }
+    } else {
-    else {
         if let Some(_) = arg.optional {
             // default value
             let mut default = Tokens::new();
             if let Some(d) = spec.default_value(name) {
-                let dt = quote!{ Some(#d) };
+                let dt = quote! { Some(#d) };
                 dt.to_tokens(&mut default);
             } else {
                 syn::Ident::from("None").to_tokens(&mut default);
@@ -500,8 +506,7 @@ fn impl_arg_param(arg: &FnArg, spec: &FnSpec, body: &Tokens, idx: usize) -> Toke
                     Err(e) => Err(e)
                 }
             }
-        }
+        } else {
-        else {
             quote! {
                 match _iter.next().unwrap().as_ref().unwrap().extract() {
                     Ok(#arg_name) => {

src/conversion.rs

@@ -301,3 +301,31 @@ impl<T> PyTryFrom for T where T: PyTypeInfo {
         }
     }
 }
+
+
+/// This trait wraps a T: IntoPyObject into PyResult<T> while PyResult<T> remains PyResult<T>.
+///
+/// This is necessaty because proc macros run before typechecking and can't decide
+/// whether a return type is a (possibly aliased) PyResult or not. It is also quite handy because
+/// the codegen is currently built on the assumption that all functions return a PyResult.
+pub trait ReturnTypeIntoPyResult {
+    type Inner: ToPyObject;
+
+    fn return_type_into_py_result(self) -> PyResult<Self::Inner>;
+}
+
+impl<T: ToPyObject> ReturnTypeIntoPyResult for T {
+    type Inner = T;
+
+    default fn return_type_into_py_result(self) -> PyResult<Self::Inner> {
+        Ok(self)
+    }
+}
+
+impl<T: ToPyObject> ReturnTypeIntoPyResult for PyResult<T> {
+    type Inner = T;
+
+    fn return_type_into_py_result(self) -> PyResult<Self::Inner> {
+        self
+    }
+}

src/err.rs

@@ -140,7 +140,7 @@ pub struct PyDowncastError;
 /// Helper conversion trait that allows to use custom arguments for exception constructor.
 pub trait PyErrArguments {
     /// Arguments for exception
-    fn arguments(&self, Python) -> PyObject;
+    fn arguments(&self, _: Python) -> PyObject;
 }
 
 impl PyErr {

src/lib.rs

@@ -165,7 +165,8 @@ pub use python::{Python, ToPyPointer, IntoPyPointer, IntoPyDictPointer};
 pub use pythonrun::{GILGuard, GILPool, prepare_freethreaded_python, prepare_pyo3_library};
 pub use instance::{PyToken, PyObjectWithToken, AsPyRef, Py, PyNativeType};
 pub use conversion::{FromPyObject, PyTryFrom, PyTryInto,
-                     ToPyObject, ToBorrowedObject, IntoPyObject, IntoPyTuple};
+                     ToPyObject, ToBorrowedObject, IntoPyObject, IntoPyTuple,
+                     ReturnTypeIntoPyResult};
 pub mod class;
 pub use class::*;
 

tests/test_class.rs

@@ -396,6 +396,11 @@ impl StaticMethod {
     fn method(py: Python) -> PyResult<&'static str> {
         Ok("StaticMethod.method()!")
     }
+
+    #[staticmethod]
+    fn no_parameters() -> PyResult<&'static str> {
+        Ok("StaticMethod.no_parameters()!")
+    }
 }
 
 #[test]
@@ -408,6 +413,7 @@ fn static_method() {
     d.set_item("C", py.get_type::<StaticMethod>()).unwrap();
     py.run("assert C.method() == 'StaticMethod.method()!'", None, Some(d)).unwrap();
     py.run("assert C().method() == 'StaticMethod.method()!'", None, Some(d)).unwrap();
+    py.run("assert C.no_parameters() == 'StaticMethod.no_parameters()!'", None, Some(d)).unwrap();
 }
 
 #[py::class]