C API Stability¶
Python’s C API is covered by the Backwards Compatibility Policy, PEP 387. While the C API will change with every minor release (e.g. from 3.9 to 3.10), most changes will be 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.
Names prefixed by an underscore, such as _Py_InternalState
,
are private API that can change without notice even in patch releases.
Stable Application Binary Interface¶
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.
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.
(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.)
-
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 ofPY_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
to3
. This works the same as0x03020000
(Python 3.2, the version that introduced 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:
PyBaseObject_Type
PyBool_Type
PyByteArrayIter_Type
PyBytesIter_Type
PyBytes_DecodeEscape()
PyBytes_Repr()
PyCFunction_Call()
PyCFunction_GetFlags()
PyCFunction_GetFunction()
PyCFunction_GetSelf()
PyCFunction_New()
PyCFunction_NewEx()
PyCFunction_Type
PyCMethod_New()
PyCapsule_Type
PyClassMethodDescr_Type
PyDictItems_Type
PyDictIterItem_Type
PyDictIterKey_Type
PyDictIterValue_Type
PyDictKeys_Type
PyDictProxy_Type
PyDictRevIterItem_Type
PyDictRevIterKey_Type
PyDictRevIterValue_Type
PyDictValues_Type
PyEllipsis_Type
PyEnum_Type
PyErr_Display()
PyErr_ProgramText()
PyEval_CallFunction()
PyEval_CallMethod()
PyEval_CallObjectWithKeywords()
PyExc_ArithmeticError
PyExc_AssertionError
PyExc_AttributeError
PyExc_BaseException
PyExc_BlockingIOError
PyExc_BrokenPipeError
PyExc_BufferError
PyExc_BytesWarning
PyExc_ChildProcessError
PyExc_ConnectionAbortedError
PyExc_ConnectionError
PyExc_ConnectionRefusedError
PyExc_ConnectionResetError
PyExc_DeprecationWarning
PyExc_EOFError
PyExc_EncodingWarning
PyExc_EnvironmentError
PyExc_Exception
PyExc_FileExistsError
PyExc_FileNotFoundError
PyExc_FloatingPointError
PyExc_FutureWarning
PyExc_GeneratorExit
PyExc_IOError
PyExc_ImportError
PyExc_ImportWarning
PyExc_IndentationError
PyExc_IndexError
PyExc_InterruptedError
PyExc_IsADirectoryError
PyExc_KeyError
PyExc_KeyboardInterrupt
PyExc_LookupError
PyExc_MemoryError
PyExc_ModuleNotFoundError
PyExc_NameError
PyExc_NotADirectoryError
PyExc_NotImplementedError
PyExc_OSError
PyExc_OverflowError
PyExc_PendingDeprecationWarning
PyExc_PermissionError
PyExc_ProcessLookupError
PyExc_RecursionError
PyExc_ReferenceError
PyExc_ResourceWarning
PyExc_RuntimeError
PyExc_RuntimeWarning
PyExc_StopAsyncIteration
PyExc_StopIteration
PyExc_SyntaxError
PyExc_SyntaxWarning
PyExc_SystemError
PyExc_SystemExit
PyExc_TabError
PyExc_TimeoutError
PyExc_TypeError
PyExc_UnboundLocalError
PyExc_UnicodeDecodeError
PyExc_UnicodeEncodeError
PyExc_UnicodeError
PyExc_UnicodeTranslateError
PyExc_UnicodeWarning
PyExc_UserWarning
PyExc_ValueError
PyExc_Warning
PyExc_WindowsError
PyExc_ZeroDivisionError
PyExceptionClass_Name()
PyFilter_Type
PyGILState_STATE
PyGetSetDescr_Type
PyListIter_Type
PyListRevIter_Type
PyLongRangeIter_Type
PyLong_GetInfo()
PyMap_Type
PyMemberDescr_Type
PyMemoryView_Type
PyMethodDescr_Type
PyModuleDef_Base
PyModuleDef_Type
PyOS_InterruptOccurred()
PyOS_mystricmp()
PyOS_mystrnicmp()
PyOS_sighandler_t
PyOS_strtol()
PyOS_strtoul()
PyObject_ClearWeakRefs()
PyObject_DelItemString()
PyObject_Format()
PyObject_SelfIter()
PyRangeIter_Type
PyRange_Type
PyReversed_Type
PySequence_In()
PySetIter_Type
PySuper_Type
PySys_HasWarnOptions()
PyThread_GetInfo()
PyThread_acquire_lock()
PyThread_acquire_lock_timed()
PyThread_allocate_lock()
PyThread_exit_thread()
PyThread_free_lock()
PyThread_get_stacksize()
PyThread_get_thread_ident()
PyThread_get_thread_native_id()
PyThread_init_thread()
PyThread_release_lock()
PyThread_set_stacksize()
PyThread_start_new_thread()
PyTraceBack_Here()
PyTraceBack_Print()
PyTraceBack_Type
PyTupleIter_Type
PyUnicodeIter_Type
PyUnicode_Append()
PyUnicode_AppendAndDel()
PyUnicode_AsDecodedObject()
PyUnicode_AsDecodedUnicode()
PyUnicode_AsEncodedObject()
PyUnicode_AsEncodedUnicode()
PyUnicode_BuildEncodingMap()
PyUnicode_DecodeCodePageStateful()
PyUnicode_FromOrdinal()
PyUnicode_GetDefaultEncoding()
PyUnicode_InternImmortal()
PyUnicode_Partition()
PyUnicode_RPartition()
PyUnicode_RSplit()
PyUnicode_Resize()
PyWeakReference
PyWrapperDescr_Type
PyZip_Type
Py_FileSystemDefaultEncodeErrors
Py_FileSystemDefaultEncoding
Py_GetRecursionLimit()
Py_HasFileSystemDefaultEncoding
Py_MakePendingCalls()
Py_SetRecursionLimit()
Py_UTF8Mode
Py_intptr_t
Py_uintptr_t
getter
setter
ssizessizeargfunc
ssizessizeobjargproc
symtable