New subject: PEP 399: Pure Python/C Accelerator Module Compatibiilty Requirements

4 Apr 2011

      At both the VM and language summits at PyCon this year, the issue of
compatibility of the stdlib amongst the various VMs came up. Two issues came
about in regards to modules that use C code. One is that code that comes in
only as C code sucks for all other VMs that are not CPython since they all
end up having to re-implement that module themselves. Two is that modules
that have an accelerator module (e.g., heapq, warnings, etc.) can end up
with compatibility options (sorry, Raymond, for picking on heapq, but is was
what bit the PyPy people most recently =).

In lieu of all of this, here is a draft PEP to more clearly state the policy
for the stdlib when it comes to C code. Since this has come up before and
this was discussed so much at the summits I have gone ahead and checked this
in so that even if this PEP gets rejected there can be a written record as
to why.

And before anyone asks, I have already run this past the lead devs of PyPy,
Jython, and IronPython and they all support what this PEP proposes. And with
the devs of the other VMs gaining push privileges there shouldn't be an
added developer burden on everyone to make this PEP happen.

==========================================================
PEP: 399
Title: Pure Python/C Accelerator Module Compatibiilty Requirements
Version: $Revision: 88219 $
Last-Modified: $Date: 2011-01-27 13:47:00 -0800 (Thu, 27 Jan 2011) $
Author: Brett Cannon 
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 04-Apr-2011
Python-Version: 3.3
Post-History:

Abstract
========

The Python standard library under CPython contains various instances
of modules implemented in both pure Python and C. This PEP requires
that in these instances that both the Python and C code *must* be
semantically identical (except in cases where implementation details
of a VM prevents it entirely). It is also required that new C-based
modules lacking a pure Python equivalent implementation get special
permissions to be added to the standard library.

Rationale
=========

Python has grown beyond the CPython virtual machine (VM). IronPython_,
Jython_, and PyPy_ all currently being viable alternatives to the
CPython VM. This VM ecosystem that has sprung up around the Python
programming language has led to Python being used in many different
areas where CPython cannot be used, e.g., Jython allowing Python to be
used in Java applications.

A problem all of the VMs other than CPython face is handling modules
from the standard library that are implemented in C. Since they do not
typically support the entire `C API of Python`_ they are unable to use
the code used to create the module. Often times this leads these other
VMs to either re-implement the modules in pure Python or in the
programming language used to implement the VM (e.g., in C# for
IronPython). This duplication of effort between CPython, PyPy, Jython,
and IronPython is extremely unfortunate as implementing a module *at
least* in pure Python would help mitigate this duplicate effort.

The purpose of this PEP is to minimize this duplicate effort by
mandating that all new modules added to Python's standard library
*must* have a pure Python implementation _unless_ special dispensation
is given. This makes sure that a module in the stdlib is available to
all VMs and not just to CPython.

Re-implementing parts (or all) of a module in C (in the case
of CPython) is still allowed for performance reasons, but any such
accelerated code must semantically match the pure Python equivalent to
prevent divergence. To accomplish this, the pure Python and C code must
be thoroughly tested with the *same* test suite to verify compliance.
This is to prevent users from accidentally relying
on semantics that are specific to the C code and are not reflected in
the pure Python implementation that other VMs rely upon, e.g., in
CPython 3.2.0, ``heapq.heappop()`` raises different exceptions
depending on whether the accelerated C code is used or not::

    from test.support import import_fresh_module

    c_heapq = import_fresh_module('heapq', fresh=['_heapq'])
    py_heapq = import_fresh_module('heapq', blocked=['_heapq'])

    class Spam:
        """Tester class which defines no other magic methods but
        __len__()."""
        def __len__(self):
            return 0

    try:
        c_heapq.heappop(Spam())
    except TypeError:
        # "heap argument must be a list"
        pass

    try:
        py_heapq.heappop(Spam())
    except AttributeError:
        # "'Foo' object has no attribute 'pop'"
        pass

This kind of divergence is a problem for users as they unwittingly
write code that is CPython-specific. This is also an issue for other
VM teams as they have to deal with bug reports from users thinking
that they incorrectly implemented the module when in fact it was
caused by an untested case.

Details
=======

Starting in Python 3.3, any modules added to the standard library must
have a pure Python implementation. This rule can only be ignored if
the Python development team grants a special exemption for the module.
Typically the exemption would be granted only when a module wraps a
specific C-based library (e.g., sqlite3_). In granting an exemption it
will be recognized that the module will most likely be considered
exclusive to CPython and not part of Python's standard library that
other VMs are expected to support. Usage of ``ctypes`` to provide an
API for a C library will continue to be frowned upon as ``ctypes``
lacks compiler guarantees that C code typically relies upon to prevent
certain errors from occurring (e.g., API changes).

Even though a pure Python implementation is mandated by this PEP, it
does not preclude the use of a companion acceleration module. If an
acceleration module is provided it is to be named the same as the
module it is accelerating with an underscore attached as a prefix,
e.g., ``_warnings`` for ``warnings``. The common pattern to access
the accelerated code from the pure Python implementation is to import
it with an ``import *``, e.g., ``from _warnings import *``. This is
typically done at the end of the module to allow it to overwrite
specific Python objects with their accelerated equivalents. This kind
of import can also be done before the end of the module when needed,
e.g., an accelerated base class is provided but is then subclassed by
Python code. This PEP does not mandate that pre-existing modules in
the stdlib that lack a pure Python equivalent gain such a module. But
if people do volunteer to provide and maintain a pure Python
equivalent (e.g., the PyPy team volunteering their pure Python
implementation of the ``csv`` module and maintaining it) then such
code will be accepted.

Any accelerated code must be semantically identical to the pure Python
implementation. The only time any semantics are allowed to be
different are when technical details of the VM providing the
accelerated code prevent matching semantics from being possible, e.g.,
a class being a ``type`` when implemented in C. The semantics
equivalence requirement also dictates that no public API be provided
in accelerated code that does not exist in the pure Python code.
Without this requirement people could accidentally come to rely on a
detail in the acclerated code which is not made available to other VMs
that use the pure Python implementation. To help verify that the
contract of semantic equivalence is being met, a module must be tested
both with and without its accelerated code as thoroughly as possible.

As an example, to write tests which exercise both the pure Python and
C acclerated versions of a module, a basic idiom can be followed::

    import collections.abc
    from test.support import import_fresh_module, run_unittest
    import unittest

    c_heapq = import_fresh_module('heapq', fresh=['_heapq'])
    py_heapq = import_fresh_module('heapq', blocked=['_heapq'])

    class ExampleTest(unittest.TestCase):

        def test_heappop_exc_for_non_MutableSequence(self):
            # Raise TypeError when heap is not a
            # collections.abc.MutableSequence.
            class Spam:
                """Test class lacking many ABC-required methods
                (e.g., pop())."""
                def __len__(self):
                    return 0

            heap = Spam()
            self.assertFalse(isinstance(heap,
                                collections.abc.MutableSequence))
            with self.assertRaises(TypeError):
                self.heapq.heappop(heap)

    class AcceleratedExampleTest(ExampleTest):

        """Test using the acclerated code."""

        heapq = c_heapq

    class PyExampleTest(ExampleTest):

        """Test with just the pure Python code."""

        heapq = py_heapq

    def test_main():
        run_unittest(AcceleratedExampleTest, PyExampleTest)

    if __name__ == '__main__':
        test_main()

Thoroughness of the test can be verified using coverage measurements
with branching coverage on the pure Python code to verify that all
possible scenarios are tested using (or not using) accelerator code.

Copyright
=========

This document has been placed in the public domain.

.. _IronPython: http://ironpython.net/
.. _Jython: http://www.jython.org/
.. _PyPy: http://pypy.org/
.. _C API of Python: http://docs.python.org/py3k/c-api/index.html
.. _sqlite3: http://docs.python.org/py3k/library/sqlite3.html

PEP 399: Pure Python/C Accelerator Module Compatibiilty Requirements

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