python-peps/pep-0232.txt

PEP: 232
Title: Function Attributes
Version: $Revision$
Author: barry@digicool.com (Barry A. Warsaw)
Status: Final
Type: Standards Track
Created: 02-Dec-2000
Python-Version: 2.1
Post-History:


Introduction

    This PEP describes an extension to Python, adding attribute
    dictionaries to functions and methods.  This PEP tracks the status
    and ownership of this feature.  It contains a description of the
    feature and outlines changes necessary to support the feature.
    This PEP summarizes discussions held in mailing list forums, and
    provides URLs for further information, where appropriate.  The CVS
    revision history of this file contains the definitive historical
    record.


Background

    Functions and methods already have a number of attributes, some of
    which are writable, e.g. func_doc, a.k.a. func.__doc__.  func_doc
    has the interesting property that there is special syntax in
    function (and method) definitions for implicitly setting the
    attribute.  This convenience has been exploited over and over
    again, overloading docstrings with additional semantics.

    For example, John Aycock has written a system where docstrings are
    used to define parsing rules[1].  Zope's ZPublisher ORB[2] uses
    docstrings to signal "publishable" methods, i.e. methods that can
    be called through the web.  And Tim Peters has developed a system
    called doctest[3], where docstrings actually contain unit tests.

    The problem with this approach is that the overloaded semantics
    may conflict with each other.  For example, if we wanted to add a
    doctest unit test to a Zope method that should not be publishable
    through the web.


Proposal

    This proposal adds a new dictionary to function objects, called
    func_dict (a.k.a. __dict__).  This dictionary can be set and get
    using ordinary attribute set and get syntax.

    Unbound methods also gain set and get attribute syntax, but they
    modify the dictionary of the underlying function object.  When
    accessed through bound methods, attributes can only be gotten;
    attempting to set an attribute on a bound method raises TypeError.
    This is because while function attributes appear to be unique to
    the bound method, they are really shared by all bound methods, via
    the underlying function object.

    A function object's __dict__ can also be set, but only to a
    dictionary object (i.e. setting __dict__ to UserDict raises a
    TypeError). 


Examples

    Here are some examples of what you can do with this feature.

        def a():
	    pass

	a.publish = 1
	a.unittest = '''...'''

	if a.publish:
	    print a()

	if hasattr(a, 'unittest'):
	    testframework.execute(a.unittest)

	class C:
	    def a(self):
	        'just a docstring'
            a.publish = 1

	c = C()
	if c.a.publish:
            publish(c.a())


Other Uses

    Paul Prescod enumerated a bunch of other uses:

    http://mail.python.org/pipermail/python-dev/2000-April/003364.html


Future Directions

    - Currently, Python supports function attributes only on Python
      functions (i.e. those that are written in Python, not those that
      are built-in).  Should it be worthwhile, a separate patch can be
      crafted that will add function attributes to built-ins.

    - __doc__ is the only function attribute that currently has
      syntactic support for conveniently setting.  It may be
      worthwhile to eventually enhance the language for supporting
      easy function attribute setting.  Here are some syntaxes
      suggested by PEP reviewers:

      def a {
          'publish' : 1,
          'unittest': '''...''',
          }
          (args):
          # ...

      def a(args):
          """The usual docstring."""
          {'publish' : 1,
           'unittest': '''...''',
           # etc.
           }

      It isn't currently clear if special syntax is necessary or
      desirable.


Dissenting Opinion

    When this was discussed on the python-dev mailing list in April
    2000, a number of dissenting opinions were voiced.  For
    completeness, the discussion thread starts here:

    http://mail.python.org/pipermail/python-dev/2000-April/003361.html

    The dissenting arguments appear to fall under the following
    categories:

    - no clear purpose (what does it buy you?)
    - other ways to do it (e.g. mappings as class attributes)
    - useless until syntactic support is included

    Countering some of these arguments is the observation that with
    vanilla Python 2.0, __doc__ can in fact be set to any type of
    object, so some semblance of writable function attributes are
    already feasible.  But that approach is yet another corruption of
    __doc__.

    And while it is of course possible to add mappings to class
    objects (or in the case of function attributes, to the function's
    module), it is more difficult and less obvious how to extract the
    attribute values for inspection.

    Finally, it may be desirable to add syntactic support, much the
    same way that __doc__ syntactic support exists.  This can be
    considered separately from the ability to actually set and get
    function attributes.


Reference Implementation

    The reference implementation is available on SourceForge as a
    patch against the Python CVS tree (patch #103123).  This patch
    doesn't include the regrtest module and output file.  Those are
    available upon request.

   http://sourceforge.net/patch/?func=detailpatch&patch_id=103123&group_id=5470

    This patch has been applied and will become part of Python 2.1.


References

    [1] Aycock, "Compiling Little Languages in Python",
    http://www.foretec.com/python/workshops/1998-11/proceedings/papers/aycock-little/aycock-little.html

    [2] http://classic.zope.org:8080/Documentation/Reference/ORB

    [3] ftp://ftp.python.org/pub/python/contrib-09-Dec-1999/System/doctest.py
    

Copyright

    This document has been placed in the Public Domain.



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