Hello. I am writing a PEP to describe how I'd like to "resurrect" and maintain the Stable ABI & Limited API going forward. I assume this'll be my main focus for CPython in 2021. I'm not as far along as I hoped to be at the end of 2020, but I guess it's time to request comments.

If you have any thoughts, arguments or improvements I'd be happy to hear them!

I'm including the text below; a rendered and forkable/pull-request-able version is at https://github.com/encukou/abi3/blob/main/PEP.rst

Have a great start of 2021!

PEP: 9999 Title: Maintaining the Stable ABI Author: Petr Viktorin <encukou@gmail.com> Discussions-To: Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 08-Dec-2020

Abstract

[A short (~200 word) description of the technical issue being addressed.]

XXX: Abstract should be written last

Motivation

:pep:384 defined a limited API and stable ABI, which allows extenders and embedders of CPython to compile extension modules that are binary-compatible with any subsequent version of 3.x. In theory, this brings many advantages:

• A module can be built only once per platform and support multiple versions of Python, reducing time, power and maintainer attention needed for builds.
• Binary wheels using the stable ABI work with new versions of CPython throughout the pre-release period, and can be tested in environments where building from source is not practical.

As a welcome side effect of the stable ABI's hiding of implementation details is that it is becoming a viable target for alternate Python implementations that need to implement (parts of) the C API.

However, in hindsignt, PEP 384 and its implementation has several issues:

• There is no process keep the ABI up to date.
• Contents of the limited API are not listed explicitly, making it unclear if a particular member (e.g. function, structure) is a part of it.
• There is no way to deprecate parts of the limited API.

This PEP defines the limited API more clearly and introducess process designed to make the API more useful.

Additionally, PEP 384 defines a *limited API* as a way to build against the stable ABI. This PEP defines the limited API more robustly.

Rationale

This PEP contains a lot of clarifications and definitions, but just one big technical change: the stable ABI will be explicitly listed in a human-maintained “manifest” file.

There have been efforts to collect such lists automatically, e.g. by scanning the symbols exported from Python. This might seem to be easier to maintain by our volunteer team.

However, designing a future-proof API is not a trivial task. The cost of updating an explicit manifest is small compared to the overall work that should go into changing API that will need to be suppported forever (or until Python 3 reaches end of life, if that comes sooner).

This PEP proposes automatically generating things *from* the manifest: initially documentation and DLL contents, with later possibilities for also automating tests.

Stable ABI vs. Limited API

:pep:384 and this document deal with the *Limited API* and the *Stable ABI*, two related but distinct concepts. This section clarifies what they mean and defines some of their semantics (either pre-existing or newly proposed here).

The word “Extensions” is used as a shorthand for all code that uses the Python API, e.g. extension modules or software that embeds Python.

Stable ABI

The CPython *Stable ABI* is a promise that extensions built with a specific Cpython version will be usable with any newer interpreter of the same major version, on the same platform and with the same compiler & settings. For example, a extension built with CPython 3.10 Stable ABI will be usable with CPython 3.11, 3.12, and so on, but not necessarily with 4.0.

The Stable ABI is not generally forward-compatible: an extension built and tested with CPython 3.10 will not generally be compatible with CPython 3.9.

.. note:: For example, starting in Python 3.10, the Py_tp_doc slot may be set to NULL, while in older versions, a NULL value will likely crash the interpreter.

The Stable ABI trades performance for its stability. For example, many functions in the stable ABI are available as faster macros to extensions that are built for a specific CPython version.

Future Python sversions may deprecate some members of the Stable ABI. Such members will still work, but may suffer from issues like reduced performance or, in the most extreme cases, memory/resource leaks.

Limited API

Stable ABI guarantee holds for extensions compiled from code that restricts itself to the *Limited API*, a subset of CPython's C API.

The limited API is used when preprocessor macro Py_LIMITED_API is defined to either 3 or the current PYTHON_API_VERSION.

The Limited API is not guaranteed to be *stable*. In the future, parts of the limited API may be deprecated. They may even be removed, as long as the *stable ABI* is kept stable and Python's general backwards compatibility policy, :pep:387, is followed.

.. note::

For example, a function declaration might be removed from public header
files but kept in the library.
This is currently a possibility for the future; this PEP does not to 

propose a concrete process for deprecations and removals.

The goal is for the limited API to cover everything needed to interact with the interpreter. There main reasons to not include a public API in the limited subset should be that it needs implementation details that change between CPython versions, like struct memory layouts, for performance reasons.

The limited API is not limited to CPython; other implementations are encouraged to implement it and help drive its design.

Specification

To make the stable ABI more useful and stable, the following changes are proposed.

Stable ABI Manifest

All members of the stable ABI – functions, typedefs, structs, struct fields, data values etc. – will be explicitly listed in a single "manifest" file, along with the Limited API version they were added in. Struct fields that users of the stable ABI are allowed to access will be listed explicitly. Members that are not part of the Limited API, but are part of the Stable ABI (e.g. PyObject.ob_type, which is accessible by the Py_TYPE macro), will be annotated as such.

Notes saying “Part of the stable ABI” will be added to Python's documentation automatically, in a way similar to the notes on functions that return borrowed references.

Source for the Windows shared library python3.dll will be generated from the stable ABI definition.

The format of the manifest will be subject to change whenever needed. It should be consumed only by scripts in the CPython repository. If a more public list is needed, a script can be added to generate it.

Contents of the Stable ABI

The initial stable ABI manifest will include:

• The Stable ABI specified in :pep:384.
• All functions listed in PC/python3dll.c.
• All structs (struct typedefs) which these functions return or take as arguments. (Fields of such structs will not necessarily be added.)
• New type slots, such as Py_am_aiter.
• The type flags Py_TPFLAGS_DEFAULT, Py_TPFLAGS_BASETYPE, Py_TPFLAGS_HAVE_GC, Py_TPFLAGS_METHOD_DESCRIPTOR.
• The calling conventions METH_* (except deprecated ones).
• All API needed by macros is the stable ABI (usually annotated as not being part of the limited API).

Additional items may be aded to the initial manifest according to the checklist below.

Testing the Stable ABI

An automatically generated test module will be added to ensure that all members of the stable ABI are available at compile time

For each function in the stable ABI, a test will be added that calls the function using ctypes. (Where calling is not practical, such as with functions related to intepreter initialization and shutdown, the test will only look the function up.) This should prevent regressions when a function is converted to a macro, which keeps the same API but breaks the ABI. An check will be added to ensure all functions in the stable ABI are tested this way.

Changing the Limited API

A checklist for changing the limited API, including new members (structs, functions or values), will be added to the Devguide_. The checklist will 1) mention best practices and common pitfalls in Python C API design and 2) guide the developer around the files that need changing and scripts that need running when the limited API is changed.

Below is the initial proposal for the checklist. After the PEP is accepted, see the Devguide for the current version.

Note that the checklist applies to new additions; not the existing limited API.

Design considerations:

• Make sure the change does not break the Stable ABI of any version of Python since 3.5.

• Make sure no exposed names are private (i.e. begin with an underscore).

• Make sure the new API is well documented.

• Make sure the types of all parameters and return values of the added function(s) and all fields of the added struct(s) are be part of the limited API (or standard C).

• Make sure the new API and its intended use follows standard C, not just features of currently suppoerted platforms.

  • Do not cast a function pointer to void* (a data pointer) or vice versa.

• Make sure the new API follows reference counting conventions. (Following them makes the API easier to reason about, and easier use in other Python implementations.)

  • Do not return borrowed references from functions.
  • Do not steal references to function arguments.

• Make sure the ownership rules and lifetimes of all applicable struct fields, arguments and return values are well defined.

• Think about ease of use for the user. (In C, ease of use itself is not very important; what *is* important is reducing boilerplate code needed to use the API. Bugs like to hide in boiler plates.)

  • If a function will be often called with specific value for an argument, consider making it default (assumed when NULL is passed in).

• Think about future extensions: for example, if it's possible that future Python versions will need to add a new field to your struct, how will that be done?

• Make as few assumptions as possible about details that might change in future CPython versions or differ across C API implementations:

  • The GIL
  • Garbage collection
  • Layout of PyObject and other structs

If following these guidelines would hurt performance, add a fast function (or macro) to the non-limited API and a stable equivalent to the limited API.

If anything is unclear, or you have a good reason to break the guidelines, consider discussing the change at the capi-sig_ mailing list.

.. _capi-sig: https://mail.python.org/mailman3/lists/capi-sig.python.org/

Procedure:

• Move the declaration to a header file directly under Include/ and #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03yy0000 (with the yy corresponding to Python version).
• Make an entry in the stable ABI list. (XXX: mention filename)
• For functions, add a test that calls the function using ctypes (XXX: mention filename).
• Regenerate the autogenerated files. (XXX: specific instructions)

Advice for Extenders and Embedders

The following notes will be added to documentation.

Extension authors should test with all Python versions they support, and preferably build with the lowest such version.

Compiling with Py_LIMITED_API defined is *not* a guarantee that your code conforms to the limited API or the stable ABI. It only covers definitions, but an API also includes other issues, such as expected semantics.

Examples of issues that Py_LIMITED_API does not guard against are:

• Calling a function with invalid arguments
• An function that started accepting NULL values for an argument in Python 3.9 will fail if NULL is passed to it under Python 3.8. Only testing with 3.8 (or lower versions) will uncover this issue.
• Some structs include a few fields that are part of the stable ABI and other fields that aren't. Py_LIMITED_API does not filter out such “private” fields.
• Using something that is not documented as part of the stable ABI, but exposed even with Py_LIMITED_API defined. Despite the team's best efforts, such issues may happen.

Backwards Compatibility

The PEP aims at full compatibility with the existing stable ABI and limited API, but defines them terms more explicitly. It might not be consistent with some interpretations of what the existing stable ABI/limited API is.

Security Implications

None known.

How to Teach This

Technical documentation will be provided. It will be aimed at experienced users familiar with C.

Reference Implementation

None so far.

Rejected Ideas

While this PEP acknowledges that parts of the limited API might be deprecated or removed in the future, a process to do this is not in scope, and is left to a possible future PEP.

Open Issues

None so far.

References

Copyright

This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.

.. _Devguide: https://devguide.python.org/

.. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: