1.. highlight:: c 2 3.. _stable: 4 5*************** 6C API Stability 7*************** 8 9Python's C API is covered by the Backwards Compatibility Policy, :pep:`387`. 10While the C API will change with every minor release (e.g. from 3.9 to 3.10), 11most changes will be source-compatible, typically by only adding new API. 12Changing existing API or removing API is only done after a deprecation period 13or to fix serious issues. 14 15CPython's Application Binary Interface (ABI) is forward- and 16backwards-compatible across a minor release (if these are compiled the same 17way; see :ref:`stable-abi-platform` below). 18So, code compiled for Python 3.10.0 will work on 3.10.8 and vice versa, 19but will need to be compiled separately for 3.9.x and 3.10.x. 20 21Names prefixed by an underscore, such as ``_Py_InternalState``, 22are private API that can change without notice even in patch releases. 23 24 25Stable Application Binary Interface 26=================================== 27 28Python 3.2 introduced the *Limited API*, a subset of Python's C API. 29Extensions that only use the Limited API can be 30compiled once and work with multiple versions of Python. 31Contents of the Limited API are :ref:`listed below <stable-abi-list>`. 32 33To enable this, Python provides a *Stable ABI*: a set of symbols that will 34remain compatible across Python 3.x versions. The Stable ABI contains symbols 35exposed in the Limited API, but also other ones – for example, functions 36necessary to support older versions of the Limited API. 37 38(For simplicity, this document talks about *extensions*, but the Limited API 39and Stable ABI work the same way for all uses of the API – for example, 40embedding Python.) 41 42.. c:macro:: Py_LIMITED_API 43 44 Define this macro before including ``Python.h`` to opt in to only use 45 the Limited API, and to select the Limited API version. 46 47 Define ``Py_LIMITED_API`` to the value of :c:data:`PY_VERSION_HEX` 48 corresponding to the lowest Python version your extension supports. 49 The extension will work without recompilation with all Python 3 releases 50 from the specified one onward, and can use Limited API introduced up to that 51 version. 52 53 Rather than using the ``PY_VERSION_HEX`` macro directly, hardcode a minimum 54 minor version (e.g. ``0x030A0000`` for Python 3.10) for stability when 55 compiling with future Python versions. 56 57 You can also define ``Py_LIMITED_API`` to ``3``. This works the same as 58 ``0x03020000`` (Python 3.2, the version that introduced Limited API). 59 60On Windows, extensions that use the Stable ABI should be linked against 61``python3.dll`` rather than a version-specific library such as 62``python39.dll``. 63 64On some platforms, Python will look for and load shared library files named 65with the ``abi3`` tag (e.g. ``mymodule.abi3.so``). 66It does not check if such extensions conform to a Stable ABI. 67The user (or their packaging tools) need to ensure that, for example, 68extensions built with the 3.10+ Limited API are not installed for lower 69versions of Python. 70 71All functions in the Stable ABI are present as functions in Python's shared 72library, not solely as macros. This makes them usable from languages that don't 73use the C preprocessor. 74 75 76Limited API Scope and Performance 77--------------------------------- 78 79The goal for the Limited API is to allow everything that is possible with the 80full C API, but possibly with a performance penalty. 81 82For example, while :c:func:`PyList_GetItem` is available, its “unsafe” macro 83variant :c:func:`PyList_GET_ITEM` is not. 84The macro can be faster because it can rely on version-specific implementation 85details of the list object. 86 87Without ``Py_LIMITED_API`` defined, some C API functions are inlined or 88replaced by macros. 89Defining ``Py_LIMITED_API`` disables this inlining, allowing stability as 90Python's data structures are improved, but possibly reducing performance. 91 92By leaving out the ``Py_LIMITED_API`` definition, it is possible to compile 93a Limited API extension with a version-specific ABI. This can improve 94performance for that Python version, but will limit compatibility. 95Compiling with ``Py_LIMITED_API`` will then yield an extension that can be 96distributed where a version-specific one is not available – for example, 97for prereleases of an upcoming Python version. 98 99 100Limited API Caveats 101------------------- 102 103Note that compiling with ``Py_LIMITED_API`` is *not* a complete guarantee that 104code conforms to the Limited API or the Stable ABI. ``Py_LIMITED_API`` only 105covers definitions, but an API also includes other issues, such as expected 106semantics. 107 108One issue that ``Py_LIMITED_API`` does not guard against is calling a function 109with arguments that are invalid in a lower Python version. 110For example, consider a function that starts accepting ``NULL`` for an 111argument. In Python 3.9, ``NULL`` now selects a default behavior, but in 112Python 3.8, the argument will be used directly, causing a ``NULL`` dereference 113and crash. A similar argument works for fields of structs. 114 115Another issue is that some struct fields are currently not hidden when 116``Py_LIMITED_API`` is defined, even though they're part of the Limited API. 117 118For these reasons, we recommend testing an extension with *all* minor Python 119versions it supports, and preferably to build with the *lowest* such version. 120 121We also recommend reviewing documentation of all used API to check 122if it is explicitly part of the Limited API. Even with ``Py_LIMITED_API`` 123defined, a few private declarations are exposed for technical reasons (or 124even unintentionally, as bugs). 125 126Also note that the Limited API is not necessarily stable: compiling with 127``Py_LIMITED_API`` with Python 3.8 means that the extension will 128run with Python 3.12, but it will not necessarily *compile* with Python 3.12. 129In particular, parts of the Limited API may be deprecated and removed, 130provided that the Stable ABI stays stable. 131 132 133.. _stable-abi-platform: 134 135Platform Considerations 136======================= 137 138ABI stability depends not only on Python, but also on the compiler used, 139lower-level libraries and compiler options. For the purposes of the Stable ABI, 140these details define a “platform”. They usually depend on the OS 141type and processor architecture 142 143It is the responsibility of each particular distributor of Python 144to ensure that all Python versions on a particular platform are built 145in a way that does not break the Stable ABI. 146This is the case with Windows and macOS releases from ``python.org`` and many 147third-party distributors. 148 149 150.. _stable-abi-list: 151 152Contents of Limited API 153======================= 154 155 156Currently, the Limited API includes the following items: 157 158.. limited-api-list:: 159