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

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 ;-)