In the interpreter its easy to restart it and fix such problems. If you don't want to restart the interpreter, there is another way to fix it:
Python 2.6.6 (r266:84292, Dec 27 2010, 00:02:40)
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> l = [1,2,3]
>>> sum(l)
6
>>> sum = 0 # oops! shadowed a builtin!
>>> sum(l)
Traceback (most recent call last):
File "", line 1, in
TypeError: 'int' object is not callable
>>> import sys
>>> sum = sys.modules['__builtin__'].sum # -- fixing sum
>>> sum(l)
6
This also comes in handy if you happened to assign a value to any other builtin, like dict or list
