Python mapping between lists for duplicates -

i have 2 parallel lists following:

list1 - list2 1 -------- 1 -------- b 1 -------- c 2 -------- d 2 -------- d 2 -------- d 2 -------- e 2 -------- 3 -------- 3 -------- l 3 -------- m 

i looking fast way in python count how many attributes of list 2 map different attributes in list 1. in example ouput like:

a maps 1, 2, 3 

use zip() pair values both lists , collections.defaultdict() produce mapping:

from collections import defaultdict  mapping = defaultdict(set) v1, v2 in zip(list1, list2):     mapping[v2].add(v1) 

now have dictionary mapping values list 2 sets containing unique values list 1; can print these match sample output with:

for v2 in sorted(mapping):     print '{} maps {}'.format(v2, ', '.join(map(str, sorted(mapping[v2])))) 

for sample input, produces:

>>> mapping defaultdict(<type 'set'>, {'a': set([1, 2, 3]), 'c': set([1]), 'b': set([1]), 'e': set([2]), 'd': set([2]), 'm': set([3]), 'l': set([3])}) >>> v2 in sorted(mapping): ...     print '{} maps {}'.format(v2, ', '.join(map(str, sorted(mapping[v2])))) ...  maps 1, 2, 3 b maps 1 c maps 1 d maps 2 e maps 2 l maps 3 m maps 3