How to make Python 2's getitem work on a class? [duplicate]

Question

How do I make item access ie. __getitem__ available on a class object in Python 2.x?

I've tried:

class B:
    @classmethod
    def __getitem__(cls, key):
        raise IndexError

test:

B[0]
# TypeError: 'classobj' object has no attribute '__getitem__'
print B.__dict__
# { ... '__getitem__': <classmethod object at 0x024F5E70>}

How do I make __getitem__ work on the class itself?

You'll have to put special methods on the metaclass instead. — Martijn Pieters
– Martijn Pieters, Commented Apr 29, 2014 at 16:13
@MartijnPieters I've failed to find that question, thanks Martin. Although my B is an old-style class, but the important thing here is the special methods I see. — n611x007
– n611x007, Commented Apr 29, 2014 at 16:15
old-style classes don't support @classmethod decorators (or any other descriptors) either. — Martijn Pieters
– Martijn Pieters, Commented Apr 29, 2014 at 16:25
@MartijnPieters I made a mistake with that then. But I wonder, what happens with my decorator here? Whether my class is still old-style and whether the decorator gets ignored. — n611x007
– n611x007, Commented Apr 29, 2014 at 16:27

Community · Accepted Answer · 2017-05-23 10:26:53Z

1

As pointed out by Martijn Pieters, one would want to define a metaclass for the special methods lookup here.

If you can use new-style class (or don't know what's that):

class Meta_B(type):
    def __getitem__(self, key):
        raise IndexError
#

class B(object):
    __metaclass__ = Meta_B
#

test

B[0]
# IndexError, as expected

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answered Apr 29, 2014 at 16:33

n611x007

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