Hi,
I would like to change the Python C API. I failed to write a single
document listing all constraints and proposing all changes that I
would to do. For example, my previous PEP 620 contains too many
changes and is too long.
Here is my attempt to focus on the bare minimum and (what I consider
as) the least controversial part: list current usages of C API and
constraints of these usages. This *informal* PEP should be the base of
future PEP changing the C API.
The current draft lives at:
https://github.com/vstinner/misc/blob/main/cpython/pep-c-api-next-level.rst
My PEP is based on HPy Next Level Manifesto written by Simon Cross:
https://github.com/hpyproject/hpy/wiki/c-api-next-level-manifesto
To reach most users of the C API, I cross-posted this email to
python-dev, capi-sig and hpy-dev.
+++++++++++++++++++++++++++++++++++++++++
Taking the Python C API to the Next Level
+++++++++++++++++++++++++++++++++++++++++
::
PEP: xxx
Title: Taking the Python C API to the Next Level
Author: Victor Stinner <vstinner(a)python.org>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 28-Sep-2021
Python-Version: 3.11
While the C API is a key of the Python popularity, it causes multiple
subtle and complex issues. There are different ways to use the C API,
each usage has its own constraints, and some constraints are exclusive.
This document lists constraints but doesn't propose changes, it only
gives vague ideas how to solve some issues. More concrete C API changes
will require writing separated PEPs.
C extensions are a key component of the Python popularity
=========================================================
The Python popularity comes from its great programming language and from
its wide collection of modules freely available on PyPI. Many of the
most popular Python modules rely directly or indirectly on C extensions
written with the C API. The Python C API is a key component of the
Python popularity.
For example, the numpy project is now a common dependency on many
scientific projects and a large part of the project is written by hand
with the C API.
**Abandoning or removing the C API** is out of question. Years ago, the
incomplete C API support was the main drawback of PyPy, since PyPy only
supported a minority of C extensions.
Today, CPython still have a similar issue. **When Cython or numpy don't
support a new Python version** (because of incompatible C API changes),
many Python projects depending on them are cannot be installed,
especially during the development phase of the next Python version.
Backward compatibility and unmaintained C extensions
====================================================
One important property of the C API is the backward compatibility.
Developers expect that if their C extension works on Python 3.10, it
will work unmodified in Python 3.11: building the C extension with
Python 3.11 should be enough.
This property is even more important for unmaintained C extensions.
Sometimes, unmaintained just means that the only maintainer is busy or
overwhelmed for a few months. Sometimes, the project has no activity for
longer than 5 years.
When an incompatible change is introduced in the C API, like removing a
function or changing a function behavior, there is a **risk of breaking
an unknown number of C extensions**.
One option can be to update old C extensions when they are built on
recent Python versions, to adapt them to incompatible changes. This
conversion is non trivial and cannot handle all kinds of incompatible
changes.
Migration plan for incompatible changes
=======================================
There should be a **sensible migration path** for large C extensions
(e.g. numpy) when incompatible changes are introduced. Whenever
possible, it should be possible to write a **single code base** compatible
with old and new Python versions.
A **compatibility layer** can be maintained externally. Cython and
numpy have their own internal compatibility layer.
There should be a way to easily pick up common errors introduced by
migrating.
One practical way to **minimize the number of broken projects** is to
attempt to check in advance if an incompatible change is going to break
popular C extensions. For broken C extensions, propose a fix and wait
until a new release includes the fix, before introducing the change in
Python. Obviously, it doesn't solve the problem of less popular C
extensions and private C extensions.
Obtain the best possible performance
====================================
There are two main reasons for writing a C extension: implement a
function which cannot be written in pure Python, or write a **C
accelerator**: rewrite the 10% of an application in C where 90% of the
CPU time is spent. About the former use case, the intent is to obtain
the best possible performance. Tradeoffs are made with portability: it
is acceptable to only support a limited number of Python versions and to
only support a limited number of Python implementations (usually only
CPython).
Cython is a good example of accelerator. It is able to support a large
number of Python versions and multiple Python implementation with
compatibility layers and ``#ifdef``. The main drawback is that it is
common that Cython is **broken by incompatible changes made at each
Python release**. It happens because Cython relies on many
implementation details.
On the other side, the **limited C API** is a small as possible,
excludes implementation details on purpose, and provides a stable ABI.
Building a C extension with the limited C API only once produces a
binary wheel package usable on many Python versions, but each platform
still requires its own binary wheel package.
Emulating the current C API is inefficient
==========================================
The PyPy project is a Python implementation written from scratch, it was
not created as a CPython fork. It made many implementation choices
different than CPython: no reference counting, moving garbage collector,
JIT compiler, etc.
To support C extensions, PyPy emulates the Python C API in its cpyext
module. When the C API access an object, cpyext has to convert the PyPy
object to a CPython object (``PyObject``). CPython objects are less
efficient than PyPy objects with the PyPy JIT compiler and conversions
from PyPy objects to CPython objects are also inefficient. PyPy has to
reimplement every single detail of the CPython implementation to be as
much compatible as possible.
The C API exposes multiple implementation details:
* Reference counting, borrowed references, stealing references.
* Objects location in memory.
* Rely on pointers for object identity: Python 3.10 adds the ``Py_Is()``
function to solve this problem.
* Expose the memory layout of Python objects as part of the API.
* Expose static types.
* Implicit execution context.
* etc.
The C API of Python 3.10 is made of around 15 000 lines of C header
files, 1500 functions and 100 structures. Supporting the full C API is a
significant amount of work.
**Freezing the C API** for a few Python releases would help other Python
implementations to catch up with the latest Python version, but it
doesn't solve the efficiency problem. Moreover, it is common that adding
a new feature to Python requires to change the C API, even if it is just
to add new functions. Not adding new features to Python for a few Python
releases is out of question.
The C API prevents optimizing CPython
=====================================
It is challenging to evolve the C API to optimize CPython without
breaking the backward compatibility. Emulating the old C API is an
option, but it is inefficient.
If everything above is achievable -- and we believe it is! -- we'll
arrive in a wonderful new future where Python implementations can
experiment with all sorts of amazing new features:
* tracing garbage collectors;
* nurseries for short-lived objects;
* sub-interpreters with separate contexts;
* specialised implementations of lists;
* removing the GIL;
* avoiding the boxing of primitive types;
* just-in-time compilation;
* ... and many other things you can imagine that we haven't!
No one can guarantee that a particular new idea will work out, but
exposing fewer implementation details via the C API will make it
possible to try many new things.
--
Night gathers, and now my watch begins. It shall not end until my death.
