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How to Invert a Dictionary in Python: Comprehensions, Defaultdict, and More

Jeremy Grifski on February 21, 2019

Article was originally posted on therenegadecoder.com where it's actively maintained. In this series, I’m putting together several articles for sm...

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Michael Dawson-Haggerty • Jul 5 '20

You can also use zip if you don't mind non-unique values being merged:

In [11]: d = {'hi': 'stuff'}

In [12]: dict(zip(d.values(), d.keys()))
Out[12]: {'stuff': 'hi'}

Jeremy Grifski • Jul 5 '20

Good point! I always hesitate with this solution because it’s not immediately clear that the two collections would maintain their order. I know they do, but it still bothers me.

Michael Dawson-Haggerty • Jul 5 '20

Yeah, the dict comprehension is the clearest way of doing this. zip is mildly faster but I agree calling keys and values separately is slightly offputting:

In [7]: d = {a: b for a,b in (np.random.random((100000, 2)) * 1e6).astype(int)}

In [8]: %timeit {v: k for k, v in d.items()}
100 loops, best of 3: 12.3 ms per loop

In [9]: %timeit dict(zip(d.values(), d.keys()))
100 loops, best of 3: 10.8 ms per loop

Niels van Galen Last • Feb 24 '19 • Edited

Inverting the original dict with non-unique values can be done cleaner (without the explicit two line for loop and IMO easier to read) using a defaultdict:

>>> from collections import defaultdict
>>> # Original dict
>>> my_dict = {
  'Izuku Midoriya': 'One for All',
  'Katsuki Bakugo': 'Explosion',
  'All Might': 'One for All',
  'Ochaco Uraraka': 'Zero Gravity'
}
>>> my_dict_inverted = defaultdict(list)

>>> [my_dict_inverted[v].append(k) for k, v in my_dict.items()]

>>> my_dict_inverted
defaultdict(<class 'list'>, {'One for All': ['Izuku Midoriya', 'All Might'],
   'Explosion': ['Katsuki Bakugo'], 'Zero Gravity': ['Ochaco Uraraka']})

Or to make it more intuitive denote the comprehension using a dict since that is what we are applying the operation to:

>>> my_dict_inverted = defaultdict(list)

>>> {my_dict_inverted[v].append(k) for k, v in my_dict.items()}

>>> my_dict_inverted
defaultdict(<class 'list'>, {'One for All': ['Izuku Midoriya', 'All Might'],
   'Explosion': ['Katsuki Bakugo'], 'Zero Gravity': ['Ochaco Uraraka']})

Jeremy Grifski • Feb 24 '19 • Edited

Great addition! I would prefer to maintain the dict type and leave out any extraneous imports, but this gets the job done and is probably less error prone.

Mind if I add it to the list?

Niels van Galen Last • Feb 24 '19

Go ahead :)

It's part of the standard library and it's still a dict so you can use any it like a normal dict, e.g. my_dict_inverted['One for All'] as well as all methods from a normal dict like keys(), values() and items().

Jeremy Grifski • May 16 '19

Hi again! I'm in the process of updating this article, and I just realized that this answer has a minor problem. We're using a dictionary comprehension in a bad way. We're updating some external dictionary while the comprehension will also generate a dictionary (in this case, {None}). I believe that's considered bad practice, so I've added a note in the original article.