C API Stability

Unless documented otherwise, Python’s C API is covered by the Backwards Compatibility Policy, PEP 387. Most changes to it are source-compatible (typically by only adding new API). Changing existing API or removing API is only done after a deprecation period or to fix serious issues.

CPython’s Application Binary Interface (ABI) is forward- and backwards-compatible across a minor release (if these are compiled the same way; see Platform Considerations below). So, code compiled for Python 3.10.0 will work on 3.10.8 and vice versa, but will need to be compiled separately for 3.9.x and 3.10.x.

There are two tiers of C API with different stability expectations:

  • Unstable API, may change in minor versions without a deprecation period. It is marked by the PyUnstable prefix in names.

  • Limited API, is compatible across several minor releases. When Py_LIMITED_API is defined, only this subset is exposed from Python.h.

These are discussed in more detail below.

Names prefixed by an underscore, such as _Py_InternalState, are private API that can change without notice even in patch releases. If you need to use this API, consider reaching out to CPython developers to discuss adding public API for your use case.

Unstable C API

Any API named with the PyUnstable prefix exposes CPython implementation details, and may change in every minor release (e.g. from 3.9 to 3.10) without any deprecation warnings. However, it will not change in a bugfix release (e.g. from 3.10.0 to 3.10.1).

It is generally intended for specialized, low-level tools like debuggers.

Projects that use this API are expected to follow CPython development and spend extra effort adjusting to changes.

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 work with 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 work without recompilation 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 compatible across Python 3.x versions.

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.

We also recommend reviewing documentation of all used API to check if it is explicitly part of the Limited API. Even with Py_LIMITED_API defined, a few private declarations are exposed for technical reasons (or even unintentionally, as bugs).

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.

Platform Considerations

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.

Contents of Limited API

Currently, the Limited API includes the following items: