guide: add type overview (#801)

* book: add type overview

fixes #789

Co-Authored-By: David Hewitt <1939362+davidhewitt@users.noreply.github.com>
Co-Authored-By: Yuji Kanagawa <yuji.kngw.80s.revive@gmail.com>

* Add concrete pointer types to the list.

* Add suggestions from @kngwyu

Co-authored-by: David Hewitt <1939362+davidhewitt@users.noreply.github.com>
Co-authored-by: Yuji Kanagawa <yuji.kngw.80s.revive@gmail.com>

guide/src/SUMMARY.md

@@ -7,10 +7,11 @@
 - [Type Conversions](conversions.md)
 - [Python Exceptions](exception.md)
 - [Calling Python from Rust](python_from_rust.md)
+- [GIL, mutability and object types](types.md)
 - [Parallelism](parallelism.md)
 - [Debugging](debugging.md)
 - [Advanced Topics](advanced.md)
 - [Building and Distribution](building_and_distribution.md)
 - [PyPy support](pypy.md)
 - [Appendix A: PyO3 and rust-cpython](rust_cpython.md)
-- [Appendix B: Migration Guide](migration.md)
+- [Appendix B: Migration Guide](migration.md)

guide/src/types.md

@@ -0,0 +1,158 @@
+# GIL lifetimes, mutability and Python object types
+
+On first glance, PyO3 provides a huge number of different types that can be used
+to wrap or refer to Python objects.  This page delves into the details and gives
+an overview of their intended meaning, with examples when each type is best
+used.
+
+
+## Mutability and Rust types
+
+Since Python has no concept of ownership, and works solely with boxed objects,
+any Python object can be referenced any number of times, and mutation is allowed
+from any reference.
+
+The situation is helped a little by the Global Interpreter Lock (GIL), which
+ensures that only one thread can use the Python interpreter and its API at the
+same time, while non-Python operations (system calls and extension code) can
+unlock the GIL.  (See [the section on parallelism](parallelism.md) for how to do
+that in PyO3.)
+
+In PyO3, holding the GIL is modeled by acquiring a token of the type
+`Python<'py>`, which serves three purposes:
+
+* It provides some global API for the Python interpreter, such as
+  [`eval`][eval].
+* It can be passed to functions that require a proof of holding the GIL,
+  such as [`PyObject::clone_ref`][clone_ref].
+* Its lifetime can be used to create Rust references that implicitly guarantee
+  holding the GIL, such as [`&'py PyAny`][PyAny].
+
+The latter two points are the reason why some APIs in PyO3 require the `py:
+Python` argument, while others don't.
+
+The PyO3 API for Python objects is written such that instead of requiring a
+mutable Rust reference for mutating operations such as
+[`PyList::append`][PyList_append], a shared reference (which, in turn, can only
+be created through `Python<'_>` with a GIL lifetime) is sufficient.
+
+However, Rust structs wrapped as Python objects (called `pyclass` types) usually
+*do* need `&mut` access.  Due to the GIL, PyO3 *can* guarantee thread-safe acces
+to them, but it cannot statically guarantee uniqueness of `&mut` references once
+an object's ownership has been passed to the Python interpreter, ensuring
+references is done at runtime using `PyCell`, a scheme very similar to
+`std::cell::RefCell`.
+
+
+## Object types
+
+### `PyObject`
+
+**Represents:** a GIL independent reference to a Python object of unspecified
+type.
+
+**Used:** Whenever you want to carry around references to "some" Python object,
+without caring about a GIL lifetime.  For example, storing Python object
+references in a Rust struct that outlives the Python-Rust FFI boundary.
+
+Can be cloned using Python reference counts with `.clone_ref()`.
+
+**Conversions:**
+
+- To `&PyAny`: `obj.as_ref(py)`
+- To `Py<ConcreteType>`: `obj.as_ref(py).extract::<Py<ConcreteType>>`
+- To `&ConcreteType` (which must be a Python native type): `obj.cast_as(py)`
+
+
+### `Py<SomeType>`
+
+**Represents:** a GIL independent reference to a Python object of known type.
+This can be a Python native type (like `PyTuple`), or a `pyclass` type
+implemented in Rust.
+
+**Used:** Like `PyObject`, but with a known inner type.
+
+**Conversions:**
+
+- To `PyObject`: `obj.to_object(py)`
+- To `&SomeType` or `&PyCell<SomeType>`: `obj.as_ref(py)`.  For `pyclass` types
+  implemented in Rust, you get a `PyCell` (see below).  For Python native types,
+  mutating operations through PyO3's API don't require `&mut` access.
+
+**Note:** `PyObject` is semantically equivalent to `Py<PyAny>` and might be
+merged with it in the future.
+
+
+### `PyAny`
+
+**Represents:** a Python object of unspecified type, restricted to a GIL
+lifetime.  Currently, `PyAny` can only ever occur as a reference, usually
+`&PyAny`.
+
+**Used:** Whenever you want to refer to some Python object only as long as
+holding the GIL.  For example, intermediate values and arguments to
+`pyfunction`s or `pymethod`s implemented in Rust where any type is allowed.
+
+**Conversions:**
+
+- To `PyObject`: `obj.to_object(py)`
+
+
+### `PyTuple`, `PyDict`, and many more
+
+**Represents:** a native Python object of known type, restricted to a GIL
+lifetime just like `PyAny`.
+
+**Used:** Whenever you want to operate with native Python types while holding
+the GIL.  Like `PyAny`, this is the most convenient form to use for function
+arguments and intermediate values.
+
+**Conversions:**
+
+- To `PyAny`: `obj.as_ref()`
+- To `Py<T>`: `Py::from(obj)`
+
+
+### `PyCell<SomeType>`
+
+**Represents:** a reference to a Rust object (instance of `PyClass`) which is
+wrapped in a Python object.  The cell part is an analog to stdlib's
+[`RefCell`][RefCell] to allow access to `&mut` references.
+
+**Used:** for accessing pure-Rust API of the instance (members and functions
+taking `&SomeType` or `&mut SomeType`) while maintaining the aliasing rules of
+Rust references.
+
+**Conversions:**
+
+- From `PyAny`: `.downcast()`
+
+
+### `PyRef<SomeType>` and `PyRefMut<SomeType>`
+
+**Represents:** reference wrapper types employed by `PyCell` to keep track of
+borrows, analog to `Ref` and `RefMut` used by `RefCell`.
+
+**Used:** while borrowing a `PyCell`.  They can also be used with `.extract()`
+on types like `Py<T>` and `PyAny` to get a reference quickly.
+
+
+
+## Related traits and types
+
+### `PyClass`
+
+This trait marks structs defined in Rust that are also usable as Python classes,
+usually defined using the `#[pyclass]` macro.
+
+### `PyNativeType`
+
+This trait marks structs that mirror native Python types, such as `PyList`.
+
+
+
+[eval]: https://docs.rs/pyo3/latest/pyo3/struct.Python.html#method.eval
+[clone_ref]: https://docs.rs/pyo3/latest/pyo3/struct.PyObject.html#method.clone_ref
+[PyAny]: https://docs.rs/pyo3/latest/pyo3/types/struct.PyAny.html
+[PyList_append]: https://docs.rs/pyo3/latest/pyo3/types/struct.PyList.html#method.append
+[RefCell]: https://doc.rust-lang.org/std/cell/struct.RefCell.html