C API and ABI Stability

A menos que documentado de outra forma, a API C do Python é coberta pela Política de Compatibilidade com versões anteriores, PEP 387. A maioria das alterações são compatíveis com a fonte (normalmente adicionando apenas uma nova API). A alteração ou remoção da API existente só é feita após um período de descontinuação ou para corrigir problemas sérios.

A Interface Binária de Aplicação (ABI) do CPython é compatível para frente e para trás através de uma versão menor (se elas forem compiladas da mesma maneira; veja Considerações da plataforma abaixo). Portanto, o código compilado para Python 3.10.0 funcionará em 3.10.8 e vice-versa, mas precisará ser compilado separadamente para 3.9.x e 3.11.x.

Existem dois níveis de API C com diferentes expectativas de estabilidade:

  • API Instável (“Unstable API”), pode mudar em versões menores sem período de depreciação. É marcado pelo prefixo PyUnstable nos nomes.

  • API Limitada (“Limited API”), é compatível em várias versões menores. Quando Py_LIMITED_API é definido, apenas este subconjunto é exposto de Python.h.

Elas são discutidas em mais detalhes abaixo.

Nomes prefixados por um sublinhado, como _Py_InternalState, são APIs privadas que podem ser alteradas sem aviso prévio, mesmo em lançamentos de correção. Se você precisa usar essa API, considere entrar em contato com os desenvolvedores do CPython para discutir a adição de uma API pública para o seu caso de uso.

API C Instável

Qualquer API nomeada com o prefixo “PyUnstable” expõe detalhes de implementação do CPython e pode mudar em cada versão menor (por exemplo, de 3.9 para 3.10) sem nenhum aviso de depreciação. No entanto, não mudará em uma versão de correção de bugs (por exemplo, de 3.10.0 para 3.10.1).

É geralmente destinado a ferramentas especializadas de baixo nível, como depuradores.

Projetos que utilizam esta API são esperados para seguir o desenvolvimento do CPython e dedicar esforço extra para se ajustar às mudanças.

Stable Application Binary Interfaces

Python’s Stable ABI allows extensions to be compatible with multiple versions of Python, without recompilation.

Nota

For simplicity, this document talks about extensions, but Stable ABI works the same way for all uses of the API – for example, embedding Python.

There are two Stable ABIs:

  • abi3, introduced in Python 3.2, is compatible with non-free-threaded builds of CPython.

  • abi3t, introduced in Python 3.15, is compatible with free-threaded builds of CPython. It has stricter API limitations than abi3.

    Adicionado na versão 3.15.0a8 (unreleased): abi3t was added in PEP 803

It is possible for an extension to be compiled for both abi3 and abi3t at the same time; the result will be compatible with both free-threaded and non-free-threaded builds of Python. Currently, this has no downsides compared to compiling for abi3t only.

Each Stable ABI is versioned using the first two numbers of the Python version. For example, Stable ABI 3.14 corresponds to Python 3.14. An extension compiled for Stable ABI 3.x is ABI-compatible with Python 3.x and above.

Extensions that target a stable ABI must only use a limited subset of the C API. This subset is known as the Limited API; its contents are listed below.

On Windows, extensions that use a Stable ABI should be linked against python3.dll rather than a version-specific library such as python39.dll. This library only exposes the relevant symbols.

On some platforms, Python will look for and load shared library files named with the abi3 or abi3t tag (for example, mymodule.abi3.so). Free-threaded interpreters only recognize the abi3t tag, while non-free-threaded ones will prefer abi3 but fall back to abi3t. Thus, extensions compatible with both ABIs should use the abi3t tag.

Python does not necessarily check that extensions it loads have compatible ABI. Extension authors are encouraged to add a check using the Py_mod_abi slot or the PyABIInfo_Check() function, but the user (or their packaging tool) is ultimately responsible for ensuring that, for example, extensions built for Stable ABI 3.10 are not installed for lower versions of Python.

All functions in Stable ABI are present as functions in Python’s shared library, not solely as macros. This makes them usable are usable from languages that don’t use the C preprocessor, including Python’s ctypes.

Compiling for Stable ABI

Nota

Build tools (such as, for example, meson-python, scikit-build-core, or Setuptools) often have a mechanism for setting macros and synchronizing them with extension filenames and other metadata. Prefer using such a mechanism, if it exists, over defining the macros manually.

The rest of this section is mainly relevant for tool authors, and for people who compile extensions manually.

Ver também

list of recommended tools in the Python Packaging User Guide

To compile for a Stable ABI, define one or both of the following macros to the lowest Python version your extension should support, in Py_PACK_VERSION format. Typically, you should choose a specific value rather than the version of the Python headers you are compiling against.

The macros must be defined before including Python.h. Since Py_PACK_VERSION is not available at this point, you will need to use the numeric value directly. For reference, the values for a few recent Python versions are:

0x30a0000  /* Py_PACK_VERSION(3.10) */
0x30b0000  /* Py_PACK_VERSION(3.11) */
0x30c0000  /* Py_PACK_VERSION(3.12) */
0x30d0000  /* Py_PACK_VERSION(3.13) */
0x30e0000  /* Py_PACK_VERSION(3.14) */
0x30f0000  /* Py_PACK_VERSION(3.15) */

When one of the macros is defined, Python.h will only expose API that is compatible with the given Stable ABI – that is, the Limited API plus some definitions that need to be visible to the compiler but should not be used directly. When both are defined, Python.h will only expose API compatible with both Stable ABIs.

Py_LIMITED_API

Target abi3, that is, non-free-threaded builds of CPython. See above for common information.

Py_TARGET_ABI3T

Target abi3t, that is, free-threaded builds of CPython. See above for common information.

Adicionado na versão 3.15.0a8 (unreleased).

Both macros specify a target ABI; the different naming style is due to backwards compatibility.

Historical note

You can also define Py_LIMITED_API as 3. This works the same as 0x03020000 (Python 3.2, the version that introduced Stable ABI).

When both are defined, Python.h may, or may not, redefine Py_LIMITED_API to match Py_TARGET_ABI3T.

On a free-threaded build – that is, when Py_GIL_DISABLED is defined – Py_TARGET_ABI3T defaults to the value of Py_LIMITED_API. This means that there are two ways to build for both abi3 and abi3t:

  • define both Py_LIMITED_API and Py_TARGET_ABI3T, or

  • define only Py_LIMITED_API and:

    • on Windows, define Py_GIL_DISABLED;

    • on other systems, use the headers of free-threaded build of Python.

Stable ABI Scope and Performance

The goal for Stable ABI is to allow everything that is possible with the full C API, but possibly with a performance penalty. Generally, compatibility with Stable ABI will require some changes to an extension’s source code.

For example, while PyList_GetItem() is available, its “unsafe” macro variant PyList_GET_ITEM() is not. The macro can be faster because it can rely on version-specific implementation details of the list object.

For another example, when not compiling for Stable ABI, some C API functions are inlined or replaced by macros. Compiling for Stable ABI disables this inlining, allowing stability as Python’s data structures are improved, but possibly reducing performance.

By leaving out the Py_LIMITED_API or Py_TARGET_ABI3T definition, it is possible to compile Stable-ABI-compatible source for a version-specific ABI. A potentially faster version-specific extension can then be distributed alongside a version compiled for Stable ABI – a slower but more compatible fallback.

Stable ABI Caveats

Note that compiling for Stable ABI is not a complete guarantee that code will be compatible with the expected Python versions. Stable ABI prevents ABI issues, like linker errors due to missing symbols or data corruption due to changes in structure layouts or function signatures. However, other changes in Python can change the behavior of extensions.

One issue that the Py_TARGET_ABI3T and Py_LIMITED_API macros do not guard against is calling a function with arguments that are invalid in a lower Python version. For example, consider a function that starts accepting NULL for an argument. In Python 3.9, NULL now selects a default behavior, but in Python 3.8, the argument will be used directly, causing a NULL dereference and crash. A similar argument works for fields of structs.

For these reasons, we recommend testing an extension with all minor Python versions it supports.

Também recomendamos revisar a documentação de todas as APIs utilizadas para verificar se ela faz parte explicitamente da API Limitada. Mesmo com a definição de Py_LIMITED_API, algumas declarações privadas são expostas por razões técnicas (ou até mesmo acidentalmente, como bugs).

Also note that while compiling with Py_LIMITED_API 3.8 means that the extension should load on Python 3.12, and compile with Python 3.12, the same source will not necessarily compile with Py_LIMITED_API set to 3.12. In general, parts of the Limited API may be deprecated and removed, provided that Stable ABI stays stable.

Considerações da plataforma

ABI stability depends not only on Python, but also on the compiler used, lower-level libraries and compiler options. For the purposes of the Stable ABIs, these details define a “platform”. They usually depend on the OS type and processor architecture

It is the responsibility of each particular distributor of Python to ensure that all Python versions on a particular platform are built in a way that does not break the Stable ABIs, or the version-specific ABIs. This is the case with Windows and macOS releases from python.org and many third-party distributors.

ABI Checking

Adicionado na versão 3.15.

Python includes a rudimentary check for ABI compatibility.

This check is not comprehensive. It only guards against common cases of incompatible modules being installed for the wrong interpreter. It also does not take platform incompatibilities into account. It can only be done after an extension is successfully loaded.

Despite these limitations, it is recommended that extension modules use this mechanism, so that detectable incompatibilities raise exceptions rather than crash.

Most modules can use this check via the Py_mod_abi slot and the PyABIInfo_VAR macro, for example like this:

PyABIInfo_VAR(abi_info);

static PyModuleDef_Slot mymodule_slots[] = {
   {Py_mod_abi, &abi_info},
   ...
};

The full API is described below for advanced use cases.

int PyABIInfo_Check(PyABIInfo *info, const char *module_name)
Parte da ABI Estável desde a versão 3.15.

Verify that the given info is compatible with the currently running interpreter.

Return 0 on success. On failure, raise an exception and return -1.

If the ABI is incompatible, the raised exception will be ImportError.

The module_name argument can be NULL, or point to a NUL-terminated UTF-8-encoded string used for error messages.

Note that if info describes the ABI that the current code uses (as defined by PyABIInfo_VAR, for example), using any other Python C API may lead to crashes. In particular, it is not safe to examine the raised exception.

Adicionado na versão 3.15.

PyABIInfo_VAR(NAME)
Parte da ABI Estável desde a versão 3.15.

Define a static PyABIInfo variable with the given NAME that describes the ABI that the current code will use. This macro expands to:

static PyABIInfo NAME = {
    1, 0,
    PyABIInfo_DEFAULT_FLAGS,
    PY_VERSION_HEX,
    PyABIInfo_DEFAULT_ABI_VERSION
}

Adicionado na versão 3.15.

type PyABIInfo
Parte da ABI Estável (incluindo todos os membros) desde a versão 3.15.
uint8_t abiinfo_major_version

The major version of PyABIInfo. Can be set to:

  • 0 to skip all checking, or

  • 1 to specify this version of PyABIInfo.

uint8_t abiinfo_minor_version

The minor version of PyABIInfo. Must be set to 0; larger values are reserved for backwards-compatible future versions of PyABIInfo.

uint16_t flags

This field is usually set to the following macro:

PyABIInfo_DEFAULT_FLAGS
Parte da ABI Estável desde a versão 3.15.

Default flags, based on current values of macros such as Py_LIMITED_API and Py_GIL_DISABLED.

Alternately, the field can be set to the following flags, combined by bitwise OR. Unused bits must be set to zero.

ABI variant – one of:

PyABIInfo_STABLE
Parte da ABI Estável desde a versão 3.15.

Specifies that Stable ABI is used.

PyABIInfo_INTERNAL

Specifies ABI specific to a particular build of CPython. Internal use only.

Free-threading compatibility – one of:

PyABIInfo_FREETHREADED
Parte da ABI Estável desde a versão 3.15.

Specifies ABI compatible with free-threaded builds of CPython. (That is, ones compiled with --disable-gil; with t in sys.abiflags)

PyABIInfo_GIL
Parte da ABI Estável desde a versão 3.15.

Specifies ABI compatible with non-free-threaded builds of CPython (ones compiled without --disable-gil).

PyABIInfo_FREETHREADING_AGNOSTIC
Parte da ABI Estável desde a versão 3.15.

Specifies ABI compatible with both free-threaded and non-free-threaded builds of CPython, that is, both abi3 and abi3t.

uint32_t build_version

The version of the Python headers used to build the code, in the format used by PY_VERSION_HEX.

This can be set to 0 to skip any checks related to this field. This option is meant mainly for projects that do not use the CPython headers directly, and do not emulate a specific version of them.

uint32_t abi_version

The ABI version.

For Stable ABI, this field should be the value of Py_LIMITED_API or Py_TARGET_ABI3T. If both are defined, use the smaller value. (If Py_LIMITED_API is 3; use Py_PACK_VERSION(3, 2) instead of 3.)

Otherwise, it should be set to PY_VERSION_HEX.

It can also be set to 0 to skip any checks related to this field.

PyABIInfo_DEFAULT_ABI_VERSION
Parte da ABI Estável desde a versão 3.15.

The value that should be used for this field, based on current values of macros such as Py_LIMITED_API, PY_VERSION_HEX and Py_GIL_DISABLED.

Adicionado na versão 3.15.

Conteúdo da API Limitada

This is the definitive list of Limited API for Python 3.15: