PEP: 384
Title: Defining a Stable ABI
Author: Martin von Löwis <martin@v.loewis.de>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 17-May-2009
Python-Version: 3.2
Post-History:

.. canonical-doc:: :ref:`python:stable` (user docs) and
                   :ref:`devguide:c-api` (development docs)

Abstract
========

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.

Rationale
=========

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would break if their code is loaded into a newer version of the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.

Specification
=============

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.

Terminology
-----------

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.

Structures
----------

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;
    int basicsize;
    int itemsize;
    unsigned 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

typedefs
--------

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
----------------------------------

By default, all functions are available, unless they are excluded
below.
Whether a function is documented or not does not matter.

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, causing
applications that use that function to break.

Excluded Functions
------------------

All functions starting with _Py are not available to applications.
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*``).

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 initializer and finalizer functions are not available
(PyByteArray_Init, PyOS_FiniInterrupts
and all functions ending in _Fini or _ClearFreeList).

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:

- Py_BEGIN_ALLOW_THREADS, Py_BLOCK_THREADS, Py_UNBLOCK_THREADS,
  Py_END_ALLOW_THREADS

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.

Linkage
-------

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 libpython3.so, and libpython3.y.so;
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]_.

References
==========

.. [1] "python3 module definition file":
   http://svn.python.org/projects/python/branches/pep-0384/PC/python3.def
.. [2] "PEP 384 branch":
   http://svn.python.org/projects/python/branches/pep-0384/
.. [3] "ABI type conversion script":
   http://svn.python.org/projects/python/branches/pep-0384/Tools/scripts/abitype.py

Copyright
=========

This document has been placed in the public domain.
