NichtJens NichtJens - 5 months ago 22
Python Question

Assigning (instead of defining) a __getitem__ magic method breaks indexing

I have a wrapper class similar to this (strongly simplified) example:

class wrap(object):
def __init__(self):
self._data = range(10)

def __getitem__(self, key):
return self._data.__getitem__(key)


I can use it like this:

w = wrap()
print w[2] # yields "2"


I thought I could optimize and get rid of one function call by changing to this:

class wrap(object):
def __init__(self):
self._data = range(10)
self.__getitem__ = self._data.__getitem__


However, I receive a


TypeError: 'wrap' object does not support indexing


for the
print w[2]
line with the latter version.

The direct call to the method, i.e.,
print w.__getitem__(2)
, works in both cases...

Why does the assignment version not allow indexing?

Answer

Special methods (essentially anything with two underscores on each end) have to be defined on the class. The internal lookup procedure for special methods completely skips the instance dict. Among other things, this is so if you do

class Foo(object):
    def __repr__(self):
        return 'Foo()'

the __repr__ method you defined is only used for instances of Foo, and not for repr(Foo).

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