Adding functions to classes after definition

Gabriel Genellina gagsl-py at yahoo.com.ar
Tue Jan 9 03:03:45 CET 2007


At Monday 8/1/2007 21:47, Gerard Brunick wrote:

>Consider:  A)
>  >>> class C(object):
>...     pass
>...
>  >>> def f(*args):
>...     print args
>...
>  >>> C.f = f
>  >>> C.f
><unbound method C.f>
>  >>> c=C()
>  >>> c.f()
>(<__main__.C object at 0x04A51170>,)
>
>And B)
>
>  >>> del c
>  >>> C.f = types.MethodType(f, None, C)
>  >>> C.f
><unbound method C.f>
>  >>> c = C()
>  >>> c.f()
>(<__main__.C object at 0x04A51290>,)
>
>I don't understand A).  It is my vague understanding, that methods are
>really properties that handle binding on attribute access, so B) should
>be the "right" way to add a method to a class after definition.

This is implemented using descriptors. A function object has a __get__ method:

py> f.__get__
<method-wrapper object at 0x00BC8990>
py> C.__dict__['f'] is f
True
py> C.f is f
False
py> C.f.im_func is f
True

So, when you assign C.f=f, nothing special happens; but when the 
function is retrieved from the class, the __get__ method is invoked, 
returning an "unbound method".

>   Why does
>A show up as a method?  Shouldn't it still just be a function?  Certainly
>when you define a class, there is some magic in the __new__ method that
>turns functions in the initial dictionary into methods, but does this still
>happen for all setattr after that?

There's nothing special in __new__ (relating to this, at least). Nor 
even when setting the attribute; the magic happens when you *get* the 
method as an attribute of the class object. Functions don't even have 
a __set__:

py> f.__set__
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
AttributeError: 'function' object has no attribute '__set__'


>Is is possible to set a class attribute
>equal to a regular (types.FunctionType) function?

Yes, that's what actually happens. It's not easy to *retrieve* it 
later without getting a MethodType.

>Any references that discuss these issues would be greatly appreciated.

Descriptors are documented somewhere... I think they came in Python 
2.2 along with new-style classes.


-- 
Gabriel Genellina
Softlab SRL 


	

	
		
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