Persistant ID generator

Let's say you want to map key->number, and every time you get a new key you give it a different number.
(Useful for mapping words to vector index in IR)

The easy way is:

from collections import defaultdict
from itertools import count

vector_index = defaultdict(count(0).next)
print vector_index["a"] # 0
print vector_index["a"] # 0
print vector_index["b"] # 1