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Title:Defining a Stable ABI
Last-Modified:2011-01-06 19:33:28 +0000 (Thu, 06 Jan 2011)
Author:Martin v. Löwis <martin at>
Type:Standards Track


Currently, each feature release introduces a new name for the Python DLL on Windows, and may cause incompatibilities for extension modules on Unix. This PEP proposes to define a stable set of API functions which are guaranteed to be available for the lifetime of Python 3, and which will also remain binary-compatible across versions. Extension modules and applications embedding Python can work with different feature releases as long as they restrict themselves to this stable ABI.


The primary source of ABI incompatibility are changes to the lay-out of in-memory structures. For example, the way in which string interning works, or the data type used to represent the size of an object, have changed during the life of Python 2.x. As a consequence, extension modules making direct access to fields of strings, lists, or tuples, would break if their code is loaded into a newer version of the interpreter without recompilation: offsets of other fields may have changed, making the extension modules access the wrong data.

In some cases, the incompatibilities only affect internal objects of the interpreter, such as frame or code objects. For example, the way line numbers are represented has changed in the 2.x lifetime, as has the way in which local variables are stored (due to the introduction of closures). Even though most applications probably never used these objects, changing them had required to change the PYTHON_API_VERSION.

On Linux, changes to the ABI are often not much of a problem: the system will provide a default Python installation, and many extension modules are already provided pre-compiled for that version. If additional modules are needed, or additional Python versions, users can typically compile them themselves on the system, resulting in modules that use the right ABI.

On Windows, multiple simultaneous installations of different Python versions are common, and extension modules are compiled by their authors, not by end users. To reduce the risk of ABI incompatibilities, Python currently introduces a new DLL name pythonXY.dll for each feature release, whether or not ABI incompatibilities actually exist.

With this PEP, it will be possible to reduce the dependency of binary extension modules on a specific Python feature release, and applications embedding Python can be made work with different releases.


The ABI specification falls into two parts: an API specification, specifying what function (groups) are available for use with the ABI, and a linkage specification specifying what libraries to link with. The actual ABI (layout of structures in memory, function calling conventions) is not specified, but implied by the compiler. As a recommendation, a specific ABI is recommended for selected platforms.

During evolution of Python, new ABI functions will be added. Applications using them will then have a requirement on a minimum version of Python; this PEP provides no mechanism for such applications to fall back when the Python library is too old.


Applications and extension modules that want to use this ABI are collectively referred to as "applications" from here on.

Header Files and Preprocessor Definitions

Applications shall only include the header file Python.h (before including any system headers), or, optionally, include pyconfig.h, and then Python.h.

During the compilation of applications, the preprocessor macro Py_LIMITED_API must be defined. Doing so will hide all definitions that are not part of the ABI.


Only the following structures and structure fields are accessible to applications:

  • PyObject (ob_refcnt, ob_type)
  • PyVarObject (ob_base, ob_size)
  • PyMethodDef (ml_name, ml_meth, ml_flags, ml_doc)
  • PyMemberDef (name, type, offset, flags, doc)
  • PyGetSetDef (name, get, set, doc, closure)
  • PyModuleDefBase (ob_base, m_init, m_index, m_copy)
  • PyModuleDef (m_base, m_name, m_doc, m_size, m_methods, m_traverse, m_clear, m_free)
  • PyStructSequence_Field (name, doc)
  • PyStructSequence_Desc (name, doc, fields, sequence)
  • PyType_Slot (see below)
  • PyType_Spec (see below)

The accessor macros to these fields (Py_REFCNT, Py_TYPE, Py_SIZE) are also available to applications.

The following types are available, but opaque (i.e. incomplete):

  • PyThreadState
  • PyInterpreterState
  • struct _frame
  • struct symtable
  • struct _node
  • PyWeakReference
  • PyLongObject
  • PyTypeObject

Type Objects

The structure of type objects is not available to applications; declaration of "static" type objects is not possible anymore (for applications using this ABI). Instead, type objects get created dynamically. To allow an easy creation of types (in particular, to be able to fill out function pointers easily), the following structures and functions are available:

typedef struct{
  int slot;    /* slot id, see below */
  void *pfunc; /* function pointer */
} PyType_Slot;

typedef struct{
  const char* name;
  const char* doc;
  int basicsize;
  int itemsize;
  int flags;
  PyType_Slot *slots; /* terminated by slot==0. */
} PyType_Spec;

PyObject* PyType_FromSpec(PyType_Spec*);

To specify a slot, a unique slot id must be provided. New Python versions may introduce new slot ids, but slot ids will never be recycled. Slots may get deprecated, but continue to be supported throughout Python 3.x.

The slot ids are named like the field names of the structures that hold the pointers in Python 3.1, with an added Py_ prefix (i.e. Py_tp_dealloc instead of just tp_dealloc):

  • tp_dealloc, tp_getattr, tp_setattr, tp_repr, tp_hash, tp_call, tp_str, tp_getattro, tp_setattro, tp_doc, tp_traverse, tp_clear, tp_richcompare, tp_iter, tp_iternext, tp_methods, tp_base, tp_descr_get, tp_descr_set, tp_init, tp_alloc, tp_new, tp_is_gc, tp_bases, tp_del
  • nb_add nb_subtract nb_multiply nb_remainder nb_divmod nb_power nb_negative nb_positive nb_absolute nb_bool nb_invert nb_lshift nb_rshift nb_and nb_xor nb_or nb_int nb_float nb_inplace_add nb_inplace_subtract nb_inplace_multiply nb_inplace_remainder nb_inplace_power nb_inplace_lshift nb_inplace_rshift nb_inplace_and nb_inplace_xor nb_inplace_or nb_floor_divide nb_true_divide nb_inplace_floor_divide nb_inplace_true_divide nb_index
  • sq_length sq_concat sq_repeat sq_item sq_ass_item sq_contains sq_inplace_concat sq_inplace_repeat
  • mp_length mp_subscript mp_ass_subscript

The following fields cannot be set during type definition: - tp_dict tp_mro tp_cache tp_subclasses tp_weaklist tp_print - tp_weaklistoffset tp_dictoffset


In addition to the typedefs for structs listed above, the following typedefs are available. Their inclusion in the ABI means that the underlying type must not change on a platform (even though it may differ across platforms).

  • Py_uintptr_t Py_intptr_t Py_ssize_t
  • unaryfunc binaryfunc ternaryfunc inquiry lenfunc ssizeargfunc ssizessizeargfunc ssizeobjargproc ssizessizeobjargproc objobjargproc objobjproc visitproc traverseproc destructor getattrfunc getattrofunc setattrfunc setattrofunc reprfunc hashfunc richcmpfunc getiterfunc iternextfunc descrgetfunc descrsetfunc initproc newfunc allocfunc
  • PyCFunction PyCFunctionWithKeywords PyNoArgsFunction PyCapsule_Destructor
  • getter setter
  • PyOS_sighandler_t
  • PyGILState_STATE
  • Py_UCS4

Most notably, Py_UNICODE is not available as a typedef, since the same Python version may use different definitions of it on the same platform (depending on whether it uses narrow or wide code units). Applications that need to access the contents of a Unicode string can convert it to wchar_t.

Functions and function-like Macros

All functions starting with _Py are not available to applications (see exceptions below). Also, all functions that expect parameter types that are unavailable to applications are excluded from the ABI, such as PyAST_FromNode (which expects a node*).

All other functions are available, unless excluded below.

Function-like macros (in particular, field access macros) remain available to applications, but get replaced by function calls (unless their definition only refers to features of the ABI, such as the various _Check macros)

ABI function declarations will not change their parameters or return types. If a change to the signature becomes necessary, a new function will be introduced. If the new function is source-compatible (e.g. if just the return type changes), an alias macro may get added to redirect calls to the new function when the applications is recompiled.

If continued provision of the old function is not possible, it may get deprecated, then removed, in accordance with PEP 7, causing applications that use that function to break.

Excluded Functions

Functions declared in the following header files are not part of the ABI:

  • bytes_methods.h
  • cellobject.h
  • classobject.h
  • code.h
  • compile.h
  • datetime.h
  • dtoa.h
  • frameobject.h
  • funcobject.h
  • genobject.h
  • longintrepr.h
  • parsetok.h
  • pyarena.h
  • pyatomic.h
  • pyctype.h
  • pydebug.h
  • pytime.h
  • symtable.h
  • token.h
  • ucnhash.h

In addition, functions expecting FILE* are not part of the ABI, to avoid depending on a specific version of the Microsoft C runtime DLL on Windows.

Module and type initalizer functions are not available (PyByteArray_Init, PyByteArray_Fini, PyBytes_Fini, PyCFunction_Fini, PyDict_Fini, PyFloat_ClearFreeList, PyFloat_Fini, PyFrame_Fini, PyList_Fini, PyMethod_Fini, PyOS_FiniInterrupts, PySet_Fini, PyTuple_Fini).

Several functions dealing with interpreter implementation details are not available:

  • PyInterpreterState_Head, PyInterpreterState_Next, PyInterpreterState_ThreadHead, PyThreadState_Next
  • Py_SubversionRevision, Py_SubversionShortBranch

PyStructSequence_InitType is not available, as it requires the caller to provide a static type object.

Py_FatalError will be moved from pydebug.h into some other header file (e.g. pyerrors.h).

The exact list of functions being available is given in the Windows module definition file for python3.dll [1].

Global Variables

Global variables representing types and exceptions are available to applications. In addition, selected global variables referenced in macros (such as Py_True and Py_False) are available.

A complete list of global variable definitions is given in the python3.def file [1]; those declared DATA denote variables.

Other Macros

All macros defining symbolic constants are available to applications; the numeric values will not change.

In addition, the following macros are available:


The Buffer Interface

The buffer interface (type Py_buffer, type slots bf_getbuffer and bf_releasebuffer, etc) has been omitted from the ABI, since the stability of the Py_buffer structure is not clear at this time. Inclusion in the ABI can be considered in future releases.

Signature Changes

A number of functions currently expect a specific struct, even though callers typically have PyObject* available. These have been changed to expect PyObject* as the parameter; this will cause warnings in applications that currently explicitly cast to the parameter type. These functions are PySlice_GetIndices, PySlice_GetIndicesEx, PyUnicode_AsWideChar, and PyEval_EvalCode.


On Windows, applications shall link with python3.dll; an import library python3.lib will be available. This DLL will redirect all of its API functions through /export linker options to the full interpreter DLL, i.e. python3y.dll.

On Unix systems, the ABI is typically provided by the python executable itself. PyModule_Create is changed to pass 3 as the API version if the extension module was compiled with Py_LIMITED_API; the version check for the API version will accept either 3 or the current PYTHON_API_VERSION as conforming. If Python is compiled as a shared library, it is installed as both, and; applications conforming to this PEP should then link to the former (extension modules can continue to link with no libpython shared object, but rather rely on runtime linking). The ABI version is symbolically available as PYTHON_ABI_VERSION.

Also on Unix, the PEP 3149 tag abi<PYTHON_ABI_VERSION> is accepted in file names of extension modules. No checking is performed that files named in this way are actually restricted to the limited API, and no support for building such files will be added to distutils due to the distutils code freeze.

Implementation Strategy

This PEP will be implemented in a branch [2], allowing users to check whether their modules conform to the ABI. To avoid users having to rewrite their type definitions, a script to convert C source code containing type definitions will be provided [3].