C API Stability

除非另有文档说明，Python 的 C API 将遵循 PEP 387 所描述的向下兼容策略。 对它的大部分改变都是源代码级兼容的（通常只会增加新的 API）。改变现有 API 或移除 API 只会在弃用期结束之后或需修复严重问题时才会发生。

CPython 的应用程序二进制接口（ABI）可以跨微版本向上和向下兼容（在以相同方式编译的情况下，参见下文 平台的考虑 一节）。因此，针对 Python 3.10.0 编译的代码将适用于 3.10.8，反之亦然，但对于 3.9.x 和 3.11.x 则需要单独编译。

存在具有不同稳定性预期的两个 C API 层次：

  • 不稳定 API，可能在次要版本中发生改变而没有弃用期。它的名称会以 PyUnstable 前缀来标记。

  • 受限 API，将会在多个次要版本间保持兼容。当定义了 Py_LIMITED_API 时，将只有这个子集会从 Python.h 对外公开。

这些将在下文中更详细地讨论。

带有一个下划线前缀的名称，如 _Py_InternalState，是可能不经通知就改变甚至是在补丁发布版中改变的私有 API。 如果你需要使用这样的 API，请考虑联系 CPython 开发团队 来讨论为你的应用场景添加公有 API。

不稳定 C API

任何名称带有 PyUnstable 前缀的 API 都将对外公开 CPython 的实现细节，并可能不加弃用警告即在次要版本中发生改变（例如从 3.9 到 3.10）。但是，它不会在问题修正发布版中改变（例如从 3.10.0 到 3.10.1）。

它通常是针对专门的，低层级的工具如调试器等。

使用此 API 的项目需要跟随 CPython 开发进程并花费额外的努力来适应改变。

Stable Application Binary Interface

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

Limited C API

Python 3.2 introduced the Limited API, a subset of Python's C API. Extensions that only use the Limited API can be compiled once and be loaded on multiple versions of Python. Contents of the Limited API are listed below.

Py_LIMITED_API

Define this macro before including Python.h to opt in to only use the Limited API, and to select the Limited API version.

Define Py_LIMITED_API to the value of PY_VERSION_HEX corresponding to the lowest Python version your extension supports. The extension will be ABI-compatible with all Python 3 releases from the specified one onward, and can use Limited API introduced up to that version.

Rather than using the PY_VERSION_HEX macro directly, hardcode a minimum minor version (e.g. 0x030A0000 for Python 3.10) for stability when compiling with future Python versions.

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

Stable ABI

To enable this, Python provides a Stable ABI: a set of symbols that will remain ABI-compatible across Python 3.x versions.

备注

The 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. See Python's Backwards Compatibility Policy (PEP 387) for details.

The Stable ABI contains symbols exposed in the Limited API, but also other ones – for example, functions necessary to support older versions of the Limited API.

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

On some platforms, Python will look for and load shared library files named with the abi3 tag (e.g. mymodule.abi3.so). It does not check if such extensions conform to a Stable ABI. The user (or their packaging tools) need to ensure that, for example, extensions built with the 3.10+ Limited API are not installed for lower versions of Python.

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

Limited API Scope and Performance

The goal for the Limited API is to allow everything that is possible with the full C API, but possibly with a performance penalty.

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.

Without Py_LIMITED_API defined, some C API functions are inlined or replaced by macros. Defining Py_LIMITED_API disables this inlining, allowing stability as Python's data structures are improved, but possibly reducing performance.

By leaving out the Py_LIMITED_API definition, it is possible to compile a Limited API extension with a version-specific ABI. This can improve performance for that Python version, but will limit compatibility. Compiling with Py_LIMITED_API will then yield an extension that can be distributed where a version-specific one is not available – for example, for prereleases of an upcoming Python version.

Limited API Caveats

Note that compiling with Py_LIMITED_API is not a complete guarantee that code conforms to the Limited API or the Stable ABI. Py_LIMITED_API only covers definitions, but an API also includes other issues, such as expected semantics.

One issue that Py_LIMITED_API does 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.

Another issue is that some struct fields are currently not hidden when Py_LIMITED_API is defined, even though they're part of the Limited API.

For these reasons, we recommend testing an extension with all minor Python versions it supports, and preferably to build with the lowest such version.

我们还建议查看所使用 API 的全部文档以检查其是否显式指明为受限 API 的一部分。即使定义了 Py_LIMITED_API，少数私有声明还是会出于技术原因（或者甚至是作为程序缺陷在无意中）被暴露出来。

Also note that the Limited API is not necessarily stable: compiling with Py_LIMITED_API with Python 3.8 means that the extension will run with Python 3.12, but it will not necessarily compile with Python 3.12. In particular, parts of the Limited API may be deprecated and removed, provided that the Stable ABI stays stable.

平台的考虑

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 ABI, 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 ABI. This is the case with Windows and macOS releases from python.org and many third-party distributors.

受限 API 的内容

Currently, the Limited API includes the following items: