"""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 in d:
val = d[key]
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 in inverse:
keys = inverse[val]
print(val, keys)
