Changing Python’s C API#

The C API is divided into three sections:

  1. The internal, private API, available with Py_BUILD_CORE defined. Ideally declared in Include/internal/. Any API named with a leading underscore is also considered private.

  2. The public C API, available when Python.h is included normally. Ideally declared in Include/cpython/.

  3. The Limited API, available with Py_LIMITED_API defined. Ideally declared directly under Include/.

Each section has higher stability & maintenance requirements, and you will need to think about more issues 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.


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.

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 are two main use cases for using such names rather than putting the definition in Include/internal/ (or directly in a .c file):

  • Internal helpers for other public API; users should not use these directly;

  • “Provisional” API, included in a Python release to test real-world usage of new API. Such names should be renamed when stabilized; preferably with a macro aliasing the old name to the new one. See “Finalizing the API” in PEP 590 for an example.

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.

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 use PyModule_AddFunctions to add functions.)

  • Add the _PyTestCapi_Init_* function to Modules/_testcapi/parts.h

  • Call the _PyTestCapi_Init_* from PyInit__testcapi in Modules/_testcapimodule.c.

  • Add the new C file to Modules/, 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.

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 the Py_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.

  • 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:

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, e.g. 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 without make, run this command directly:

    ./python ./Tools/scripts/ --generate-all ./Misc/stable_abi.toml
  • Build Python and check the using make check-limited-abi. On platforms without make, run this command directly:

    ./python ./Tools/scripts/ --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 its Py_LIMITED_API version is too low, add a version postfix, e.g. yourfeature_limited_3_12.c for Python 3.12+.

  • #define Py_LIMITED_API to the minimum limited API version needed.

  • #include "parts.h" after the Py_LIMITED_API definition

  • Enclose 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 decorator for Python tests in Lib/test, so they’re skipped on incompatible builds.