Reverse elements in dictionary

Question

Possible Duplicate:
Python reverse / inverse a mapping

Say I have the following.

D = {'a':1,'b':2,'c':3}

How would I reverse each element so I get

inverseD = {1:'a',2:'b',3'c'}

irrelevant, but I still don't understand those people who downvote a user's legit confusion on some topic, even given his/her stack life has only been a month. — nye17, May 24 '12 at 01:33
Agreed. A downvote for a newcomers question should at least be accompanied by an explanation for the downvote and/or a suggestion on how to improve the question. — Joel Cornett, May 24 '12 at 01:44

score 7 · Accepted Answer · edited May 24 '12 at 01:11

7

use a dict comprehension (Python 2.7+ and 3.0+):

D = {'a':1,'b':2,'c':3}
inverse = {v: k for k, v in D.items()}
print(inverse)
# {1: 'a', 2: 'b', 3: 'c'}

edited May 24 '12 at 01:11

jamylak

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answered May 24 '12 at 01:05

mata

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heh, exactly same answer. I had to go through the captcha check, that's why I'm late. I'll delete my answer. +1. – ch3ka May 24 '12 at 01:08

mhawke · Answer 2 · 2012-05-24T01:18:57.717

For Python 2.6 and earlier (no dict comprehension):

d = {'a':1,'b':2,'c':3}
inverse_dict = dict((v,k) for k,v in d.items())

This assumes that the values of dict d are hashable, for example, it won't work on this dictionary:

>>> d={'l':[],'d':{},'t':()}
>>> inverse_dict = dict((v,k) for k,v in d.items())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

You will also run into problems if the values of the original dict are not unique, e.g.

>>> d={'a': 1, 'c': 3, 'b': 2, 'd': 1}
>>> dict((v,k) for k,v in d.items())
{1: 'd', 2: 'b', 3: 'c'}

Exactly what I needed. Thanks. Couldn't find it in the manual. — user1352521, May 24 '12 at 01:11

score 3 · Answer 3 · answered May 24 '12 at 01:12

You can use mata's answer if you have Python2.7+ otherwise use mhawke's answer.

Inverting a dict like this only works properly if all the values of the source dict are unique and hashable

If the values are hashable, but not unique you can make a dict with having lists for the values instead

Reverse elements in dictionary

3 Answers3