Reverse / invert a dictionary mapping

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Chapters
00:00 Reverse / Invert A Dictionary Mapping
00:15 Answer 1 Score 51
00:43 Answer 2 Score 232
00:58 Accepted Answer Score 1505
01:12 Answer 4 Score 209
01:27 Thank you

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Full question
https://stackoverflow.com/questions/4836...

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Content licensed under CC BY-SA
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Tags
#python #dictionary #mapping #reverse

#avk47

ACCEPTED ANSWER

Score 1505

Python 3+:

inv_map = {v: k for k, v in my_map.items()}

Python 2:

inv_map = {v: k for k, v in my_map.iteritems()}

ANSWER 2

Score 232

Assuming that the values in the dict are unique:

Python 3:

dict((v, k) for k, v in my_map.items())

Python 2:

dict((v, k) for k, v in my_map.iteritems())

ANSWER 3

Score 209

If the values in my_map aren't unique:

Python 3:

inv_map = {}
for k, v in my_map.items():
    inv_map[v] = inv_map.get(v, []) + [k]

Python 2:

inv_map = {}
for k, v in my_map.iteritems():
    inv_map[v] = inv_map.get(v, []) + [k]

ANSWER 4

Score 51

Try this:

inv_map = dict(zip(my_map.values(), my_map.keys()))

(Note that the Python docs on dictionary views explicitly guarantee that .keys() and .values() have their elements in the same order, which allows the approach above to work.)

Alternatively:

inv_map = dict((my_map[k], k) for k in my_map)

or using python 3.0's dict comprehensions

inv_map = {my_map[k] : k for k in my_map}