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This is because y is a new-style class (because file is) and z is not.
If the return type of new-style classe's len is not an int, Python tries
to convert it, which is why that works for the first example. However,
that conversion doesn't happen in old-style classes. Try:
>>> class z(object):
... def __init__(self, name):
... self.f = file(name, 'r')
... def __len__(self):
... self.f.seek(0, 2)
... return self.f.tell()
>>> x = z('/tmp/longfile')
>>> len(x)
[Whatever it is]