Why aren't colons optional?

[Alex Martelli]

Roy Smith wrote:

> Alex Martelli <aleax at aleax.it> wrote:
> 
>> No, it would have substantially impoverished the language:
>> 
>> class doubleface:
>>     def __getitem__(self, name):
>>         return "anitem"
>>     def __call__(self, *args):
>>         return "acall"
> 
> Can you give me a practical example of why somebody would want to define a
> __getitem__ method for a class?

Up to 2.1 (or for non-new-type classes today) it's for example the only
way to enable iteration on instances.  But it has of course many uses
even now that __iter__ has appeared for that purpose.  For example:

class manyfuncs(list):
    def __call__(self, *args, **kwds):
        return [func(*args, **kwds) for func in self]

a trivially-simple application of the Multiplexer design pattern (here I 
don't need to override the __getitem__ I inherit from list, but it would
be a tragedy if my __call__ was deemed to clash with it!).

Example use:

import math
trig = manyfuncs([math.sin, math.cos, math.tan])

Now I have a "function" (callable instance) trig which given any number
as argument returns a list of three results about that number.  Of course
I can manipulate trig also as a list-of-functions (actually, of callables)
with indexing, slicing and so on (if I wanted to make trig immutable I
might have inherited from tuple instead, but even then I could still have
wanted to _examine_ the i-th function in the tuple, so __getitem__
remains obviously useful).


Alex