PEP: 232 Title: Function Attributes Version: $Revision$ Last-Modified:
$Date$ Author: Barry Warsaw <barry@python.org> Status: Final Type:
Standards Track Content-Type: text/x-rst Created: 02-Dec-2000
Python-Version: 2.1 Post-History: 20-Feb-2001

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 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.

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.

Methods also gain getter syntax, and they currently access the attribute
through the dictionary of the underlying function object. It is not
possible to set attributes on bound or unbound methods, except by doing
so explicitly on the underlying function object. See the Future
Directions discussion below for approaches in subsequent versions of
Python.

A function object's __dict__ can also be set, but only to a dictionary
object. Deleting a function's __dict__, or setting it to anything other
than a concrete dictionary object results in a TypeError. If no function
attributes have ever been set, the function's __dict__ will be empty.

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 on the python-dev thread.

Future Directions

Here are a number of future directions to consider. Any adoption of
these ideas would require a new PEP, which referenced this one, and
would have to be targeted at a Python version subsequent to the 2.1
release.

-   A previous version of this PEP allowed for both setter and getter of
    attributes on unbound methods, and only getter on bound methods. A
    number of problems were discovered with this policy.

    Because method attributes were stored in the underlying function,
    this caused several potentially surprising results:

        class C:
            def a(self): pass

        c1 = C()
        c2 = C()
        c1.a.publish = 1
        # c2.a.publish would now be == 1 also!

    Because a change to a bound c1 also caused a change to a bound to
    c2, setting of attributes on bound methods was disallowed. However,
    even allowing setting of attributes on unbound methods has its
    ambiguities:

        class D(C): pass
        class E(C): pass

        D.a.publish = 1
        # E.a.publish would now be == 1 also!

    For this reason, the current PEP disallows setting attributes on
    either bound or unbound methods, but does allow for getting
    attributes on either -- both return the attribute value on the
    underlying function object.

    A future PEP might propose to implement setting (bound or unbound)
    method attributes by setting attributes on the instance or class,
    using special naming conventions. I.e.:

        class C:
            def a(self): pass

        C.a.publish = 1
        C.__a_publish__ == 1 # true

        c = C()
        c.a.publish = 2
        c.__a_publish__ == 2 # true

        d = C()
        d.__a_publish__ == 1 # true

    Here, a lookup on the instance would look to the instance's
    dictionary first, followed by a lookup on the class's dictionary,
    and finally a lookup on the function object's dictionary.

-   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:[3]

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

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

        def a(args) having (publish = 1):
            # see reference [3]
            pass

    The BDFL is currently against any such special syntactic support for
    setting arbitrary function attributes. Any syntax proposals would
    have to be outlined in new PEPs.

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 on python-dev.

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

This PEP has been accepted and the implementation has been integrated
into Python 2.1.

References

Copyright

This document has been placed in the public domain.

[1] Aycock, "Compiling Little Languages in Python",
https://legacy.python.org/workshops/1998-11/proceedings/papers/aycock-little/aycock-little.html

[2] https://web.archive.org/web/20010307022153/http://classic.zope.org:8080/Documentation/Reference/ORB

[3] Hudson, Michael, SourceForge patch implementing this syntax,
https://web.archive.org/web/20010901050535/http://sourceforge.net/tracker/index.php?func=detail&aid=403441&group_id=5470&atid=305470