Method / Functions - What are the differences?

Alf P. Steinbach alfps at start.no
Sun Feb 28 15:08:49 CET 2010


* Michael Rudolf:
> Out of curiosity I tried this and it actually worked as expected:
> 
>  >>> class T(object):
>     x=[]
>     foo=x.append
>     def f(self):
>         return self.x
> 
>     
>  >>> t=T()
>  >>> t.f()
> []
>  >>> T.foo(1)
>  >>> t.f()
> [1]
>  >>>
> 
> At first I thought "hehe, always fun to play around with python. Might 
> be useful sometimes" - but then It really confused me what I did. I 
> mean: f is what we call a method, right? But was is foo?

foo is (refers to) an object that supports call notation and that forwards calls 
somewhere else, in this case to append on a list.

You might call it (descriptive) a call forwarder, or (C# or general terminology) 
a delegate, or (Python 2.x) a bound method.


<example>
   >>> "Hello".upper
   <built-in method upper of str object at 0x00BA16E0>
   >>> f = "Hello".upper
   >>> f
   <built-in method upper of str object at 0x00BA16E0>
   >>> f()
   'HELLO'
   >>>
   >>>
   >>>
   >>> f.__self__
   'Hello'
   >>> f.__call__
   <method-wrapper '__call__' of builtin_function_or_method object at 0x00BDD170>
   >>> print( f.__doc__ )
   S.upper() -> str

   Return a copy of S converted to uppercase.
   >>> _
</example>


A common use for delegates is as command handlers in a GUI application, and in 
general for event notifications.


Cheers & hth.,

- Alf



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