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Merge pull request #3525 from davidhewitt/architecture-corrections 

small tidy ups to Architecture.md
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@ -24,17 +24,18 @@ To summarize, there are six main parts to the PyO3 codebase.
- [`src/instance.rs`] and [`src/types`]
3. [`PyClass` and related functionalities.](#3-pyclass-and-related-functionalities)
- [`src/pycell.rs`], [`src/pyclass.rs`], and more
4. [Protocol methods like `__getitem__`.](#4-protocol-methods)
- [`src/class`]
5. [Procedural macros to simplify usage for users.](#5-procedural-macros-to-simplify-usage-for-users)
- [`src/derive_utils.rs`], [`pyo3-macros`] and [`pyo3-macros-backend`]
6. [`build.rs` and `pyo3-build-config`](#6-buildrs-and-pyo3-build-config)
4. [Procedural macros to simplify usage for users.](#4-procedural-macros-to-simplify-usage-for-users)
- [`src/impl_`], [`pyo3-macros`] and [`pyo3-macros-backend`]
5. [`build.rs` and `pyo3-build-config`](#5-buildrs-and-pyo3-build-config)
- [`build.rs`](https://github.com/PyO3/pyo3/tree/main/build.rs)
- [`pyo3-build-config`]
## 1. Low-level bindings of Python/C API
[`pyo3-ffi`] contains wrappers of [Python/C API].
[`pyo3-ffi`] contains wrappers of the [Python/C API]. This is currently done by hand rather than
automated tooling because:
- it gives us best control about how to adapt C conventions to Rust, and
- there are many Python interpreter versions we support in a single set of files.
We aim to provide straight-forward Rust wrappers resembling the file structure of
[`cpython/Include`](https://github.com/python/cpython/tree/v3.9.2/Include).
@ -44,12 +45,12 @@ the file contents upstream in CPython.
The tracking issue is [#1289](https://github.com/PyO3/pyo3/issues/1289), and contribution is welcome.
In the [`pyo3-ffi`] crate, there is lots of conditional compilation such as `#[cfg(Py_LIMITED_API)]`,
`#[cfg(Py_37)]`, and `#[cfg(PyPy)]`.
`#[cfg(Py_3_7)]`, and `#[cfg(PyPy)]`.
`Py_LIMITED_API` corresponds to `#define Py_LIMITED_API` macro in Python/C API.
With `Py_LIMITED_API`, we can build a Python-version-agnostic binary called an
[abi3 wheel](https://pyo3.rs/latest/building_and_distribution.html#py_limited_apiabi3).
`Py_37` means that the API is available from Python >= 3.7.
There are also `Py_38`, `Py_39`, and so on.
`Py_3_7` means that the API is available from Python >= 3.7.
There are also `Py_3_8`, `Py_3_9`, and so on.
`PyPy` means that the API definition is for PyPy.
Those flags are set in [`build.rs`](#6-buildrs-and-pyo3-build-config).
@ -104,26 +105,16 @@ Since we need lots of boilerplate for implementing common traits for these types
traits to make `#[pyclass]` work.
Also, [`src/pyclass_init.rs`] and [`src/impl_/pyclass.rs`] have related functionalities.
To realize object-oriented programming in C, all Python objects must have the following two fields
at the beginning.
```rust
#[repr(C)]
pub struct PyObject {
pub ob_refcnt: usize,
pub ob_type: *mut PyTypeObject,
...
}
```
Thanks to this guarantee, casting `*mut A` to `*mut PyObject` is valid if `A` is a Python object.
To realize object-oriented programming in C, all Python objects have `ob_base: PyObject` as their
first field in their structure definition. Thanks to this guarantee, casting `*mut A` to `*mut PyObject`
is valid if `A` is a Python object.
To ensure this guarantee, we have a wrapper struct `PyCell<T>` in [`src/pycell.rs`] which is roughly:
```rust
#[repr(C)]
pub struct PyCell<T: PyClass> {
object: crate::ffi::PyObject,
ob_base: crate::ffi::PyObject,
inner: T,
}
```
@ -142,7 +133,7 @@ In Python, all objects have their types, and types are also objects of `type`.
For example, you can see `type({})` shows `dict` and `type(type({}))` shows `type` in Python REPL.
`T: PyTypeInfo` implies that `T` has a corresponding type object.
## 4. Protocol methods
### Protocol methods
Python has some built-in special methods called dunder methods, such as `__iter__`.
They are called "slots" in the [abstract objects layer](https://docs.python.org/3/c-api/abstract.html) in
@ -151,15 +142,15 @@ We provide a way to implement those protocols similarly, by recognizing special
names in `#[pymethods]`, with a few new ones for slots that can not be
implemented in Python, such as GC support.
## 5. Procedural macros to simplify usage for users.
## 4. Procedural macros to simplify usage for users.
[`pyo3-macros`] provides five proc-macro APIs: `pymodule`, `pyfunction`, `pyclass`,
`pymethods`, and `#[derive(FromPyObject)]`.
[`pyo3-macros-backend`] has the actual implementations of these APIs.
[`src/derive_utils.rs`] contains some utilities used in code generated by these proc-macros,
[`src/impl_`] contains `#[doc(hidden)]` functionality used in code generated by these proc-macros,
such as parsing function arguments.
## 6. `build.rs` and `pyo3-build-config`
## 5. `build.rs` and `pyo3-build-config`
PyO3 supports a wide range of OSes, interpreters and use cases. The correct environment must be
detected at build time in order to set up relevant conditional compilation correctly. This logic
@ -218,7 +209,7 @@ Some of the functionality of `pyo3-build-config`:
<!-- Files -->
[`src/derive_utils.rs`]: https://github.com/PyO3/pyo3/blob/main/src/derive_utils.rs
[`src/impl_`]: https://github.com/PyO3/pyo3/blob/main/src/impl_
[`src/instance.rs`]: https://github.com/PyO3/pyo3/tree/main/src/instance.rs
[`src/pycell.rs`]: https://github.com/PyO3/pyo3/tree/main/src/pycell.rs
[`src/pyclass.rs`]: https://github.com/PyO3/pyo3/tree/main/src/pyclass.rs