Search for a key by its value in a dictionary in Python

This post will discuss how to search a key by its value in dictionary in Python.

1. Using Generator Expression

A simple solution is to use a generator expression to search a key by its value in a dictionary. Here’s what the code would look like:

# create dictionary {'A': 65, 'B': 66, 'C': 67, … , 'Z': 90}

asciiMap = {chr(_): _ for _ in range(65, 91)}

# value to search

val = 75

# prints K

print(next(ch for ch, code in asciiMap.items() if code == val))

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2. Using Inverse Dictionary

Another option is to create a dictionary of value-key pairs. We can use the zip() function to bundle together the dictionary’s values and keys and pass the result to the dictionary constructor to get a dictionary.

# create dictionary {'A': 65, 'B': 66, 'C': 67, … , 'Z': 90}

asciiMap = {chr(_): _ for _ in range(65, 91)}

# value to search

val = 75

reverseMap = dict(zip(asciiMap.values(), asciiMap.keys()))

print(reverseMap.get(val)) # prints K

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This assumes no two keys in the dictionary have the same value, and all dictionary values are hashable. We can simplify the code with the map() function:

# create dictionary {'A': 65, 'B': 66, 'C': 67, … , 'Z': 90}

asciiMap = {chr(_): _ for _ in range(65, 91)}

# value to search

val = 75

reverseMap = dict(map(reversed, asciiMap.items()))

print(reverseMap.get(val)) # prints K

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3. Using for loop

We can even use a for-loop to search for a key by its value by looping over the items of a dictionary and checking if the value matches the given value. For example:

# create dictionary {'A': 65, 'B': 66, 'C': 67, … , 'Z': 90}

asciiMap = {chr(_): _ for _ in range(65, 91)}

# value to search

val = 75

for k, v in asciiMap.items():

if v == val:

print(k) # prints K

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4. Using list comprehension

Finally, to get a list of all keys that match the given value, we can use list comprehension. The list comprehension will create a new list of keys that are selected from the dictionary based on a condition. For example, we can search for all the keys having the value 1 in the dictionary using a list comprehension like this:

# create dictionary {'A': 1, 'B': 1, 'C': 1, 'D': 1, 'E': 1}

d = {chr(_): 1 for _ in range(65, 70)}

# value to search

val = 1

# Create an empty list to store the keys

keys = [k for k, v in d.items() if v == val]

print(keys) # ['A', 'B', 'C', 'D', 'E']

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That’s all about searching for a key by its value in a dictionary in Python.

Search for a key by its value in a dictionary in Python

1. Using Generator Expression

2. Using Inverse Dictionary

3. Using for loop

4. Using list comprehension

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