Reverse / invert a dictionary mapping

Question

Given a dictionary like so:

my_map = {\'a\': 1, \'b\': 2}

How can one invert this map to get:

inv_map = {1: \'a\', 2: \'b\'         


        

Answer 1

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}

Answer 2

Combination of list and dictionary comprehension. Can handle duplicate keys

{v:[i for i in d.keys() if d[i] == v ] for k,v in d.items()}

Answer 3

This handles non-unique values and retains much of the look of the unique case.

inv_map = {v:[k for k in my_map if my_map[k] == v] for v in my_map.itervalues()}

For Python 3.x, replace itervalues with values.

Answer 4

A lambda solution for current python 3.x versions:

d1 = dict(alice='apples', bob='bananas')
d2 = dict(map(lambda key: (d1[key], key), d1.keys()))
print(d2)

Result:

{'apples': 'alice', 'bananas': 'bob'}

This solution does not check for duplicates.

Some remarks:

• The lambda construct can access d1 from the outer scope, so we only pass in the current key. It returns a tuple.
• The dict() constructor accepts a list of tuples. It also accepts the result of a map, so we can skip the conversion to a list.
• This solution has no explicit for loop. It also avoids using a list comprehension for those who are bad at math ;-)

Answer 5

To do this while preserving the type of your mapping (assuming that it is a dict or a dict subclass):

def inverse_mapping(f):
    return f.__class__(map(reversed, f.items()))

Answer 6

I found that this version is more than 10% faster than the accepted version of a dictionary with 10000 keys.

d = {i: str(i) for i in range(10000)}

new_d = dict(zip(d.values(), d.keys()))