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Compare:
>>> class y(file):
... def __init__(self, name):
... file.__init__(self, name)
... def __len__(self):
... self.seek(0, 2)
... return self.tell()
... 
>>> n = y('/tmp/longfile')
>>> len(n)
364773376
Versus:
>>> class z:
... 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)
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: __len__() should return an int
Because:
>>> x = file('/tmp/longfile')
>>> type(x.tell())
<type 'long'>

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]

Thanks Benjamin, that is a very interesting feature that I can't find
documented anywhere. Is there perhaps a documentation bug that can arise
from this? There are various places where the differences between old
and new-style classes are documented to a greater or lesser degree, eg:
http://www.python.org/download/releases/2.2.3/bugs/
Is this documented somewhere currently that I missed, or could it be
incorporated somewhere?
thanks,
D

The documentation is still not very good about documenting the
differences between new and old style classes. Perhaps this is something
which could go under the __len__ entry in "Special Method Names."