collections.Counter (Python 2.7 only!)
It is designed to keep "tallies" or count instances of something. The example will make it all clear:
from collections import Counter cars = Counter() # I see one go past, it is red cars['red'] += 1 # And a green one cars['blue'] += 1 # etc
This is pretty much like a defaultdict with an integer value, but it is convenient and neat, with some useful constructors. Don't you think?