[Python-Dev] PEP 384: Defining a Stable ABI

["Martin v. Löwis"]

Thomas Wouters reminded me of a long-standing idea; I finally
found the time to write it down.

Please comment!

Regards,
Martin

PEP: 384
Title: Defining a Stable ABI
Version: $Revision: 72754 $
Last-Modified: $Date: 2009-05-17 21:14:52 +0200 (So, 17. Mai 2009) $
Author: Martin v. Löwis <martin at v.loewis.de>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 17-May-2009
Python-Version: 3.2
Post-History:

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would break if their code is loaded into a newer version of the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)
- Py_buffer (buf, obj, len, itemsize, readonly, ndim, shape,
  strides, suboffsets, smalltable, internal)
- PyMethodDef (ml_name, ml_meth, ml_flags, ml_doc)
- PyMemberDef (name, type, offset, flags, doc)
- PyGetSetDef (name, get, set, doc, closure)

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

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;

  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_print, 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_set, 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 was_sq_ass_slice
  sq_contains sq_inplace_concat sq_inplace_repeat
- mp_length mp_subscript mp_ass_subscript
- bf_getbuffer bf_releasebuffer

XXX Not supported yet: tp_weaklistoffset, tp_dictoffset

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

Functions and function-like Macros
----------------------------------

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*``).

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:
- cellobject.h
- classobject.h
- code.h
- frameobject.h
- funcobject.h
- genobject.h
- pyarena.h
- pydebug.h
- symtable.h
- token.h
- traceback.h

Global Variables
----------------

Global variables representing types and exceptions are available
to applications.
XXX provide a complete list.

XXX should restrict list of globals to truly "builtin" stuff,
excluding everything that can also be looked up through imports.

XXX may specify access to predefined types and exceptions through
the interpreter state, with appropriate Get macros.

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

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.

XXX is it possible to redirect global variables in the same way?
If not, python3.dll would have to copy them, and we should verify
that all available global variables are read-only.

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.

XXX is it possible to make the soname libpython.so.3, and still
have some applications link to libpython3.y.so?

Implementation Strategy
=======================

This PEP will be implemented in a branch, allowing users to check
whether their modules conform to the ABI. To simplify this testing, an
additional macro Py_LIMITED_API_WITH_TYPES will expose the existing
type object layout, to let users postpone rewriting all types. When
the this branch is merged into the 3.2 code base, this macro will
be removed.

Copyright
=========

This document has been placed in the public domain.