PEP 232 – Function Attributes

PEP: 232
Title: Function Attributes
Author: Barry Warsaw <barry at python.org>
Status: Final
Type: Standards Track
Created: 02-Dec-2000
Python-Version: 2.1
Post-History: 20-Feb-2001

Introduction
Background
Proposal
Examples
- Other Uses
Future Directions
Dissenting Opinion
Reference Implementation
References
Copyright

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 4.

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:
```
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 5.

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.