How to do Obj-C Categories in Python?

Question

Obj-C (which I have not used for a long time) has something called categories to extend classes. Declaring a category with new methods and compiling it into your program, all in

Answer 1

I came up with this implementation of a class decorator. I'm using python2.5 so I haven't actually tested it with decorator syntax (which would be nice), and I'm not sure what it does is really correct. But it looks like this:

pycategories.py

"""
This module implements Obj-C-style categories for classes for Python

Copyright 2009 Ulrik Sverdrup <ulrik.sverdrup@gmail.com>
License: Public domain
"""

def Category(toclass, clobber=False):
    """Return a class decorator that implements the decorated class'
    methods as a Category on the class @toclass

    if @clobber is not allowed, AttributeError will be raised when
    the decorated class already contains the same attribute.
    """
    def decorator(cls):
        skip = set(("__dict__", "__module__", "__weakref__", "__doc__"))
        for attr in cls.__dict__:
            if attr in toclass.__dict__:
                if attr in skip:
                    continue
                if not clobber:
                    raise AttributeError("Category cannot override %s" % attr)
            setattr(toclass, attr, cls.__dict__[attr])
        return cls
    return decorator

Answer 2

Python's setattr function makes this easy.

# categories.py

class category(object):
    def __init__(self, mainModule, override = True):
        self.mainModule = mainModule
        self.override = override

    def __call__(self, function):
        if self.override or function.__name__ not in dir(self.mainModule):
            setattr(self.mainModule, function.__name__, function)

 

# categories_test.py

import this
from categories import category

@category(this)
def all():
    print "all things are this"

this.all()
>>> all things are this

Answer 3

Why not just add methods dynamically ?

>>> class Foo(object):
>>>     pass
>>> def newmethod(instance):
>>>     print 'Called:', instance
...
>>> Foo.newmethod = newmethod
>>> f = Foo()
>>> f.newmethod()
Called: <__main__.Foo object at 0xb7c54e0c>

I know Objective-C and this looks just like categories. The only drawback is that you can't do that to built-in or extension types.