__getitem__ method on (meta)classes

[Bengt Richter]

On Mon, 14 Mar 2005 22:00:38 -0800, Ron Garret <rNOSPAMon at flownet.com> wrote:

>In article <39n1jpF60pc7pU1 at individual.net>,
> Leif K-Brooks <eurleif at ecritters.biz> wrote:
>
>> ron at flownet.com wrote:
>> > Why doesn't this work?
>> > 
>> > 
>> >>>>def foo(lst):
>> > 
>> > ...   class baz(object):
>> > ...     def __getitem__(cls, idx): return cls.lst[idx]
>> > ...     __getitem__=classmethod(__getitem__)
>> > ...   baz.lst = lst
>> > ...   return baz
>> > ...
>> > 
>> > I thought x[y] and x.__getitem__(y) were supposed to always be
>> > synonymous.
>> 
>> No, with new-style classes, x[y] and type(x).__getitem__(y) are 
>> synonymous.
>
>Ah.
>
>Did you mean type(x).__getitem__(x,y)?
>
Not if x is a classmethod, since type(x).__getitem__ gets you a bound-to-the-class method
instead of the usual unbound method, which would want the x instance as the first argument.

 >>> def foo(lst):
 ...    class baz(object):
 ...      def __getitem__(cls, idx): return cls.lst[idx]
 ...      __getitem__=classmethod(__getitem__)
 ...    baz.lst = lst
 ...    return baz
 ...
 >>> f = foo([1,2,3])()
 >>> type(f).__getitem__
 <bound method type.__getitem__ of <class '__main__.baz'>>
 >>> type(f).__getitem__(0)
 1

Leaving out the classmethod:

 >>> def foo(lst):
 ...    class baz(object):
 ...      def __getitem__(cls, idx): return cls.lst[idx]
 ...    baz.lst = lst
 ...    return baz
 ...
 >>> f = foo([1,2,3])()
 >>> type(f).__getitem__
 <unbound method baz.__getitem__>
 >>> type(f).__getitem__(f, 0)
 1

>And where is this documented?
Between the lines in my previous post ;-)


Regards,
Bengt Richter