Python Language Tutorial => Searching key(s) for a value in dict

Example

dict have no builtin method for searching a value or key because dictionaries are unordered. You can create a function that gets the key (or keys) for a specified value:

def getKeysForValue(dictionary, value):
    foundkeys = []
    for keys in dictionary:
        if dictionary[key] == value:
            foundkeys.append(key)
    return foundkeys

This could also be written as an equivalent list comprehension:

def getKeysForValueComp(dictionary, value): 
    return [key for key in dictionary if dictionary[key] == value]

If you only care about one found key:

def getOneKeyForValue(dictionary, value):
    return next(key for key in dictionary if dictionary[key] == value)

The first two functions will return a list of all keys that have the specified value:

adict = {'a': 10, 'b': 20, 'c': 10}
getKeysForValue(adict, 10)     # ['c', 'a'] - order is random could as well be ['a', 'c']
getKeysForValueComp(adict, 10) # ['c', 'a'] - dito
getKeysForValueComp(adict, 20) # ['b']
getKeysForValueComp(adict, 25) # []

The other one will only return one key:

getOneKeyForValue(adict, 10)   # 'c'  - depending on the circumstances this could also be 'a'
getOneKeyForValue(adict, 20)   # 'b'

and raise a StopIteration-Exception if the value is not in the dict:

getOneKeyForValue(adict, 25)

StopIteration

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Python Language Searching Searching key(s) for a value in dict

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