defaultdict

Python 2.5 has a defaultdict dictionary in the collections
module.
defaultdict takes a factory function in the constructor. This function
will create the default value each time you try to get a missing item.

Then you can write a word histogram function like this:

from collections import defaultdict
def histogram(text):
   histogram = defaultdict(int) # int() -> 0
   for word in text.split():
       histogram[word] += 1
   return histogram

Or, if you want to store the location of the words as well

def histogram(text):
   histogram = defaultdict(list) # list() -> []
   for location, word in enumerate(text.split()):
       histogram[word].append(location)
   return histogram