Swap keys and values in a Python dictionary

A dictionary is a one to one mapping. Every key has a value.
In Python that can be

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

You can then get the values like this

print(dict['b'])

That's great and all. But what if you want to flip the entire dictionary? Where keys are values and values and keys.

Swap values

You can easily swap the key,dictionary pairs in Python, with a one liner.

dict = {value:key for key, value in dict.items()}

So in practice you can have something like this:

dict = {'a': 1, 'b': 2, 'c': 3}
print(dict)

dict = {value:key for key, value in dict.items()}
print(dict)

This will output

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

So the one liner

dict = {value:key for key, value in dict.items()}

flipped the whole dictionary. Nice trick :)

Learn Python?

• https://pythonbasics.org/
• https://pythonprogramminglanguage.com/

Comments

[rhymes]

Keep in mind that his can fail if the value is non hashable.

>>> dict = { 1: [2] }
>>> {value:key for key, value in dict.items()}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <dictcomp>
TypeError: unhashable type: 'list'

:-)

All mutable data structures in Python are not hashable