python-peps/peps/pep-0286.rst

PEP: 286
Title: Enhanced Argument Tuples
Author: Martin von Löwis <martin@v.loewis.de>
Status: Deferred
Type: Standards Track
Content-Type: text/x-rst
Created: 03-Mar-2002
Python-Version: 2.3
Post-History:


Abstract
========

``PyArg_ParseTuple`` is confronted with difficult memory management if
an argument converter creates new memory.  To deal with these
cases, a specialized argument type is proposed.


PEP Deferral
============

Further exploration of the concepts covered in this PEP has been deferred
for lack of a current champion interested in promoting the goals of the
PEP and collecting and incorporating feedback, and with sufficient
available time to do so effectively.

The resolution of this PEP may also be affected by the resolution of
:pep:`426`, which proposes the use of a preprocessing step to generate
some aspects of C API interface code.


Problem description
===================

Today, argument tuples keep references to the function arguments,
which are guaranteed to live as long as the argument tuple exists
which is at least as long as the function call is being executed.

In some cases, parsing an argument will allocate new memory, which
is then to be released by the caller.  This has two problems:

1. In case of failure, the application cannot know what memory to
   release; most callers don't even know that they have the
   responsibility to release that memory.  Example for this are
   the ``N`` converter (bug #416288 [1]_) and the ``es#`` converter (bug
   #501716 [2]_).

2. Even for successful argument parsing, it is still inconvenient
   for the caller to be responsible for releasing the memory.  In
   some cases, this is unnecessarily inefficient.  For example,
   the ``es`` converter copies the conversion result into memory, even
   though there already is a string object that has the right
   contents.


Proposed solution
=================

A new type 'argument tuple' is introduced.  This type derives from
tuple, adding an ``__dict__`` member (at ``tp_dictoffset`` -4).  Instances
of this type might get the following attributes:

- 'failobjects', a list of objects which need to be deallocated
  in case of success

- 'okobjects', a list of object which will be released when the
  argument tuple is released

To manage this type, the following functions will be added, and
used appropriately in ``ceval.c`` and ``getargs.c``:

- ``PyArgTuple_New(int);``
- ``PyArgTuple_AddFailObject(PyObject*, PyObject*);``
- ``PyArgTuple_AddFailMemory(PyObject*, void*);``
- ``PyArgTuple_AddOkObject(PyObject*, PyObject*);``
- ``PyArgTuple_AddOkMemory(PyObject*, void*);``
- ``PyArgTuple_ClearFailed(PyObject*);``

When argument parsing fails, all fail objects will be released
through ``Py_DECREF``, and all fail memory will be released through
``PyMem_Free``.  If parsing succeeds, the references to the fail
objects and fail memory are dropped, without releasing anything.

When the argument tuple is released, all ok objects and memory
will be released.

If those functions are called with an object of a different type,
a warning is issued and no further action is taken; usage of the
affected converters without using argument tuples is deprecated.


Affected converters
===================

The following converters will add fail memory and fail objects: ``N``,
``es``, ``et``, ``es#``, ``et#`` (unless memory is passed into the converter)


New converters
==============

To simplify Unicode conversion, the ``e*`` converters are duplicated
as ``E*`` converters (``Es``, ``Et``, ``Es#``, ``Et#``).  The usage of the ``E*``
converters is identical to that of the ``e*`` converters, except that
the application will not need to manage the resulting memory.
This will be implemented through registration of Ok objects with
the argument tuple.  The ``e*`` converters are deprecated.


References
==========

.. [1] infrequent memory leak in pyexpat
   (http://bugs.python.org/issue416288)

.. [2] "es#" parser marker leaks memory
   (http://bugs.python.org/issue501716)


Copyright
=========

This document has been placed in the public domain.