Apply a function to each item of a Python dictionary

This post will discuss how to apply a function to each item of dictionary in Python.

We need to apply a mapping function f(x) to a dictionary { k₁: v₁, k₂: v₂, … , k_n: v_n} which should result in dictionary { k₁: f(v₁), k₂: f(v₂), … , k_n: f(v_n)}.

1. Using Dictionary Comprehension

A simple and fairly efficient way to apply a mapping function to each key/value pair of a dictionary is with dictionary comprehension, as shown below:

def f(x):

return x + 1

if __name__ == '__main__':

d = {'A': 0, 'B': 1, 'C': 2}

dict = {k: f(v) for k, v in d.items()}

print(dict) # {'A': 1, 'B': 2, 'C': 3}

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2. Using for-loop

If you need to perform the mapping in-place rather than creating a new dictionary, you can do so with a simple for-loop. This is demonstrated below:

def f(x):

return x + 1

if __name__ == '__main__':

d = {'A': 0, 'B': 1, 'C': 2}

for k, v in d.items():

d[k] = f(v)

print(d) # {'A': 1, 'B': 2, 'C': 3} # {'A': 1, 'B': 2, 'C': 3}

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3. Using `toolz.valmap()` function

The toolz library offers several utility functions for iterators, functions, and dictionaries. You can use the toolz.valmap() function to apply a function to the dictionary’s values. Similarly, to apply function to keys of a dictionary, use toolz.keymap() function and to apply function to items of a dictionary, use toolz.itemmap() function.

import toolz

def f(x):

return x + 1

if __name__ == '__main__':

d = {'A': 0, 'B': 1, 'C': 2}

dict = toolz.valmap(f, d)

print(dict) # {'A': 1, 'B': 2, 'C': 3}

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4. Using `map()` function

The in-built map() function provides a convenient way to apply a function to every item of iterable. You can use it for applying a mapping to the dictionary as well. You can use this as:

def f(x):

return x + 1

if __name__ == '__main__':

d = {'A': 0, 'B': 1, 'C': 2}

dict = dict(map(lambda x: (x[0], f(x[1])), d.items()))

print(dict) # {'A': 1, 'B': 2, 'C': 3}

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That’s all about applying a function to each item of the dictionary in Python.

Apply a function to each item of a Python dictionary

1. Using Dictionary Comprehension

2. Using for-loop

3. Using toolz.valmap() function

4. Using map() function

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3. Using `toolz.valmap()` function

4. Using `map()` function