Python what is the best way to get the key from dict by value (in terms of runtime speed)

Question

So let’s suppose I have the following dictionary:

data = {
    'A': ['A1', 'A2', 'A3'],
    'B': ['B1', 'B2', 'B3'],
    }

The question is what is the best way to get the key of a given value. For instance, I want to get a key in dictionary which one’s value contains A2:

def get_key_depending_on_value(value):
    for key in data.keys():
        if value in data[key]:
            return key
    return 'There is no such string in data's values'

result = get_key_depending_on_value('A2')
print(result) # prints A

I could have written such a function, but is it the best practice or python has something built in?

Asked By: Python Django

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Answer 1

I suggest you generate a new dictionary:

data = {
    'A': ['A1', 'A2', 'A3'],
    'B': ['B1', 'B2', 'B3'],
}
d = {}
for key, values in data.items():
    d.update({value: key for value in values})

Now you can just do d[value] to get the key.

Answered By: Code-Apprentice

Python what is the best way to get the key from dict by value (in terms of runtime speed)

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