"""This module contains a code example related to

Think Python, 2nd Edition

by Allen Downey

http://thinkpython2.com

Copyright 2015 Allen Downey

License: http://creativecommons.org/licenses/by/4.0/

"""

from __future__ import print_function, division

def invert_dict(d):

"""Inverts a dictionary, returning a map from val to a list of keys.

If the mapping key->val appears in d, then in the new dictionary

val maps to a list that includes key.

d: dict

Returns: dict

"""

inverse = {}

for key, val in d.items():

inverse.setdefault(val, []).append(key)

return inverse

if __name__ == '__main__':

d = dict(a=1, b=2, c=3, z=1)

inverse = invert_dict(d)

for val, keys in inverse.items():

print(val, keys)