Changing Python’s C API¶
The C API is divided into these tiers:
The internal, private API, available with
Py_BUILD_CORE
defined. Ideally declared inInclude/internal/
. Any API named with a leading underscore is also considered private.The Unstable C API, identified by the
PyUnstable_
name prefix. Ideally declared in Include/cpython/ along with the general public API.The “general” public C API, available when Include/Python.h is included normally. Ideally declared in
Include/cpython/
.The Limited C API, available with
Py_LIMITED_API
defined. Ideally declared directly underInclude/
.
Each tier has different stability and maintenance requirements to consider when you add or change definitions in it.
The compatibility guarantees for public C API are explained in the
user documentation, Doc/c-api/stable.rst
(C API Stability).
The internal API¶
Internal API is defined in Include/internal/
and is only available
for building CPython itself, as indicated by a macro like Py_BUILD_CORE
.
While internal API can be changed at any time, it’s still good to keep it stable: other API or other CPython developers may depend on it. For users, internal API is sometimes the best workaround for a thorny problem — though those use cases should be discussed on the C API Discourse category or an issue so we can try to find a supported way to serve them.
With PyAPI_FUNC or PyAPI_DATA¶
Functions or structures in Include/internal/
defined with
PyAPI_FUNC
or PyAPI_DATA
are internal functions which are
exposed only for specific use cases like debuggers and profilers.
Ideally, these should be migrated to the Unstable C API.
With the extern keyword¶
Functions in Include/internal/
defined with the extern
keyword
must not and can not be used outside the CPython code base. Only
built-in stdlib extensions (built with the Py_BUILD_CORE_BUILTIN
macro defined) can use such functions.
When in doubt, new internal C functions should be defined in
Include/internal
using the extern
keyword.
Private names¶
Any API named with a leading underscore is also considered internal.
There is currently only one main use case for using such names rather than
putting the definition in Include/internal/ (or directly in a .c
file):
Internal helpers for other public APIs, which users should not call directly.
Note that historically, underscores were used for APIs that are better served by the Unstable C API:
“provisional” APIs, included in a Python release to test real-world usage of new APIs;
APIs for very specialized uses like JIT compilers.
Internal API tests¶
C tests for the internal C API live in Modules/_testinternalcapi.c
.
Functions named test_*
are used as tests directly.
Python parts of the tests live in various places in Lib/test
.
Public C API¶
CPython’s public C API is available when Python.h
is included normally
(that is, without defining macros to select the other variants).
It should be defined in Include/cpython/
(unless part of the Limited API,
see below).
Guidelines for expanding/changing the public API¶
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.)
Functions must not steal references
Functions must not return borrowed references
Functions returning references must return a strong reference
Make sure the ownership rules and lifetimes of all applicable struct fields, arguments and return values are well defined.
Functions returning
PyObject *
must return a valid pointer on success, andNULL
with an exception raised on error. Most other API must return-1
with an exception raised on error, and0
on success.APIs with lesser and greater results must return
0
for the lesser result, and1
for the greater result. Consider a lookup function with a three-way return:return -1
: internal error or API misuse; exception raisedreturn 0
: lookup succeeded; no item was foundreturn 1
: lookup succeeded; item was found
Please start a public discussion if these guidelines won’t work for your API.
Note
By return value, we mean the value returned by the C return statement.
C API tests¶
Tests for the public C API live in the _testcapi
module.
Functions named test_*
are used as tests directly.
Tests that need Python code (or are just easier to partially write in Python)
live in Lib/test
, mainly in Lib/test/test_capi.
Due to its size, the _testcapi
module is defined in several source
files.
To add a new set of tests (or extract a set out of the monolithic
Modules/_testcapimodule.c):
Create a C file named
Modules/_testcapi/yourfeature.c
The file should define a module as usual, except:
Instead of
<Python.h>
, include"parts.h"
.Instead of
PyInit_modname
, define a_PyTestCapi_Init_yourfeature
function that takes the_testcapi
module and adds functions/classes to it. (You can usePyModule_AddFunctions
to add functions.)
Add the
_PyTestCapi_Init_*
function toModules/_testcapi/parts.h
Call the
_PyTestCapi_Init_*
fromPyInit__testcapi
inModules/_testcapimodule.c
.Add the new C file to Modules/Setup.stdlib.in, PCbuild/_testcapi.vcxproj and PCbuild/_testcapi.vcxproj.filters, alongside the other
_testcapi/*.c
entries.
Note that all Modules/_testcapi/*.c
sources initialize the same module,
so be careful about name collisions.
When moving existing tests, feel free to replace TestError
with
PyExc_AssertionError
unless actually testing custom exceptions.
Unstable C API¶
The unstable C API tier is meant for extensions that need tight integration with the interpreter, like debuggers and JIT compilers. Users of this tier may need to change their code with every feature release.
In many ways, this tier is like the general C API:
it’s available when
Python.h
is included normally,it should be defined in Include/cpython/,
it requires tests, so we don’t break it unintentionally
it requires docs, so both we and the users, can agree on the expected behavior,
it is tested and documented in the same way.
The differences are:
Names of functions structs, macros, etc. start with the
PyUnstable_
prefix. This defines what’s in the unstable tier.The unstable API can change in feature releases, without any deprecation period.
A stability note appears in the docs. This happens automatically, based on the name (via Doc/tools/extensions/c_annotations.py).
Despite being “unstable”, there are rules to make sure third-party code can use this API reliably:
Changes and removals can be done in feature releases (
3.x.0
, including Alphas and Betas for3.x.0
).Adding a new unstable API for an existing feature is allowed even after Beta feature freeze, up until the first Release Candidate. Consensus on the Core Development Discourse is needed in the Beta period.
Backwards-incompatible changes should make existing C callers fail to compile. For example, arguments should be added/removed, or a function should be renamed.
When moving an API into or out of the Unstable tier, the old name should continue to be available (but deprecated) until an incompatible change is made. In other words, while we’re allowed to break calling code, we shouldn’t break it unnecessarily.
Moving an API from the public tier to Unstable¶
Expose the API under its new name, with the
PyUnstable_
prefix. ThePyUnstable_
prefix must be used for all symbols (functions, macros, variables, etc.).Make the old name an alias (for example, a
static inline
function calling the new function).Deprecate the old name, typically using
Py_DEPRECATED
.Announce the change in the “What’s New”.
The old name should continue to be available until an incompatible change is made. Per Python’s backwards compatibility policy (PEP 387), this deprecation needs to last at least two releases (modulo Steering Council exceptions).
The rules are relaxed for APIs that were introduced in Python versions before 3.12, when the official Unstable tier was added. You can make an incompatible change (and remove the old name) as if the function was already part of the Unstable tier for APIs introduced before Python 3.12 that are either:
Documented to be less stable than default.
Named with a leading underscore.
Moving an API from the private tier to unstable¶
Expose the API under its new name, with the
PyUnstable_
prefix.If the old name is documented, or widely used externally, make it an alias and deprecate it (typically with
Py_DEPRECATED
). It should continue to be available until an incompatible change is made, as if it was previously public.This applies even to underscored names. Python wasn’t always strict with the leading underscore.
Announce the change in What’s New.
Moving an API from unstable to public¶
Expose the API under its new name, without the
PyUnstable_
prefix.Make the old
PyUnstable_*
name be an alias (for example, astatic inline
function calling the new function).Announce the change in What’s New.
The old name should remain available until the new public name is deprecated or removed. There’s no need to deprecate the old name (it was unstable to begin with), but there’s also no need to break working code just because some function is now ready for a wider audience.
Limited API¶
The Limited API is a subset of the C API designed to guarantee ABI
stability across Python 3 versions.
Defining the macro Py_LIMITED_API
will limit the exposed API to
this subset.
No changes that break the Stable ABI are allowed.
The Limited API should be defined in Include/
, excluding the
cpython
and internal
subdirectories.
Guidelines for changing the Limited API, and removing items from it¶
While the Stable ABI must not be broken, the existing Limited API can be changed, and items can be removed from it, if:
the Backwards Compatibility Policy (PEP 387) is followed, and
the Stable ABI is not broken – that is, extensions compiled with Limited API of older versions of Python continue to work on newer versions of Python.
This is tricky to do and requires careful thought. Some examples:
Functions, structs etc. accessed by macros in any version of the Limited API are part of the Stable ABI, even if they are named with an underscore. They must not be removed and their signature must not change. (Their implementation may change, though.)
Structs members cannot be rearranged if they were part of any version of the Limited API.
If the Limited API allows users to allocate a struct directly, its size must not change.
Exported symbols (functions and data) must continue to be available as exported symbols. Specifically, a function can only be converted to a
static inline
function (or macro) if Python also continues to provide the actual function. For an example, see thePy_NewRef
macro and redefinition in 3.10.
It is possible to remove items marked as part of the Stable ABI, but only if there was no way to use them in any past version of the Limited API.
Guidelines for adding to the Limited API¶
Guidelines for the general Public C API apply. See Guidelines for expanding/changing the public API.
New Limited API should only be defined if
Py_LIMITED_API
is set to the version the API was added in or higher. (See below for the proper#if
guard.)All parameter types, return values, struct members, etc. need to be part of the Limited API.
Functions that deal with
FILE*
(or other types with ABI portability issues) should not be added.
Think twice when defining macros.
Macros should not expose implementation details
Functions must be exported as actual functions, not (only) as functions-like macros.
If possible, avoid macros. This makes the Limited API more usable in languages that don’t use the C preprocessor.
Please start a public discussion before expanding the Limited API
The Limited API and must follow standard C, not just features of currently supported platforms. The exact C dialect is described in PEP 7.
Documentation examples (and more generally: the intended use of the API) should also follow standard C.
In particular, do not cast a function pointer to
void*
(a data pointer) or vice versa.
Think about ease of use for the user.
In C, ease of use itself is not very important; what is useful 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 (used when
NULL
is passed in).The Limited API needs to be well documented.
Think about future extensions
If it’s possible that future Python versions will need to add a new field to your struct, make sure it can be done.
Make as few assumptions as possible about implementation details that might change in future CPython versions or differ across C API implementations. The most important CPython-specific implementation details involve:
The GIL
Memory layout of PyObject, lists/tuples and other structures
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.
Adding a new definition to the Limited API¶
Add the declaration to a header file directly under
Include/
, into a block guarded with the following:#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x03yy0000
with the
yy
corresponding to the target CPython version, for example,0x030A0000
for Python 3.10.Append an entry to the Stable ABI manifest,
Misc/stable_abi.toml
Regenerate the autogenerated files using
make regen-limited-abi
. On platforms withoutmake
, run this command directly:./python ./Tools/build/stable_abi.py --generate-all ./Misc/stable_abi.toml
Build Python and check the using
make check-limited-abi
. On platforms withoutmake
, run this command directly:./python ./Tools/build/stable_abi.py --all ./Misc/stable_abi.toml
Add tests – see below.
Limited API tests¶
Since Limited API is a subset of the C API, there’s no need to test the behavior of individual functions. Rather, the tests could verify that some task is possible using the exposed subset, or exercise a feature that was removed from the current Limited API but still needs to be supported for older Limited API/Stable ABI versions.
To add a test file:
Add a C file
Modules/_testcapi/yourfeature_limited.c
. If that file already exists but itsPy_LIMITED_API
version is too low, add a version postfix, for example,yourfeature_limited_3_12.c
for Python 3.12+.#define Py_LIMITED_API
to the minimum limited API version needed.#include "parts.h"
after thePy_LIMITED_API
definitionEnclose the entire rest of the file in
#ifdef LIMITED_API_AVAILABLE
, so it’s skipped on incompatible builds.Follow the general instructions for C API tests. All additions go in the sections guarded by
#ifdef LIMITED_API_AVAILABLE
.
Use the test.support.requires_limited_api
decorator for Python tests
in Lib/test
, so they’re skipped on incompatible builds.