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'>