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Associating multiple values with each key in a dictionary. (Python recipe) by Michael Chermside
ActiveState Code (http://code.activestate.com/recipes/52219/)

A normal dictionary performs a mapping of the form: key -> value. Suppose you want a dictionary which maps each key to multiple values: key -> [value*]. Here's an easy and efficient way to achieve it. We rely on the setdefault() method of dictionarys both initialize the list of values for this key if necessary, and give us the list at the same time.

Python, 32 lines
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d = {}

# To add a key->value pair, do this:
d.setdefault(key, []).append(value)

# To retrieve a list of the values for a key
list_of_values = d[key]

# To remove a key->value pair is still easy, if
# you don't mind leaving empty lists behind when
# the last value for a given key is removed:
d[key].remove(value)

# Despite the empty lists, it's still possible to 
# test for the existance of values easily:
if d.has_key(key) and d[key]:
    pass # has some values for key
else:
    pass # no values for key


# But be warned... this version allows each value
# to be present multiple times:
example = {}
example.setdefault('a', []).append('apple')
example.setdefault('b', []).append('boots')
example.setdefault('c', []).append('cat')
example.setdefault('a', []).append('ant')
example.setdefault('a', []).append('apple')
# NOTE: now example['a'] == ['apple', 'ant', 'apple']
example['a'].remove('apple')
# NOTE: it's still true that ('apple' in example['a'])