Get key by value in dictionary

Keys, values and Python too

Python dictionaries are like Pandora's boxes, they store key-value pairs, but a layered look at the box can save the day:

# I need this value... but which key does it open?
key_index = list(my_dict.values()).index(my_value)
key = list(my_dict.keys())[key_index]


Important: make sure the value exists to avoid walks down the error lane.

Duplicates: There can only be one

If you find yourself dealing with keys with duplicate values, adapt your code as follows:

# Hello there, clones!
keys = [key for key, value in my_dict.items() if value == my_value]


Remember, it's like the Highlander, if the values weren't born unique, the dictionary inversion will end badly.

Avoid the banana skins

KeyError's worst enemy

Getting a KeyError is like stepping on a hidden banana skin. Fortunately, we have the try-except blocks:

try:
    # Am I lucky today?
    key = next(key for key, value in my_dict.items() if value == my_value)
except StopIteration:
    # Apparently not
    key = None


Memory: handle with care

Large dictionaries make Python slower than a turtle on sleeping pills. But iteritems() can be your memory-efficient best friend.

Readability: thy name is Python

When it comes to code, clear is better than clever. Use apt variable names and avoid reserved keywords.

Mix and match methods

Shinzu's Reverse Blade

For distinct values and intense value-to-key lookup, you might consider turning your dictionary into a two-faced beast, also known as an inverted dictionary:

key_and_value_swapped = dict(zip(my_dict.values(), my_dict.keys()))


Don't forget: this method works only on unique values.

Big Data's nightmare

In the realm of big data, the abyss of performance issues is usually lurking. But fret not! Python, with cProfile, can keep the abyss at bay:

import cProfile
cProfile.run('next((key for key, value in my_dict.items() if value == my_value), None)')


Playing Safe

Use next()'s default parameter to shield your code from StopIteration errors:

# Time to play safe
safe_key = next((key for key, value in my_dict.items() if value == my_value), None)


Life is a lot smoother when your code doesn't stumble on non-existent keys.