overriding builtins

I fired up python and wrote following code:

sum(range(5))  
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable

why did this happen? Well, I accidentally overwritten the sum builtin:

sum = 0

The fix is quite trivial: reload the builtin :)

from __builtin__ import sum