如何从指定值的字典中获取所有键？

Question

If I have dict like this:

some_dict = {'a': 1, 'b': 2, 'c': 2}

How to get keys that have values 2, like this:

some_dict.search_keys(2)

This is example. Assume some_dict is has many thousands or more keys.

Answer 1

You can do it like this:

[key for key, value in some_dict.items() if value == 2]

This uses a list comprehension to iterate through the pairs of (key, value) items, selecting those keys whose value equals 2.

Note that this requires a linear search through the dictionary, so it is O(n) . If this performance is not acceptable, you will probably need to create and maintain another data structure that indexes your dictionary by value.

Answer 2

you can also use dictionary comprehension, if you want result to be dictionary

{ x:y for x,y in some_dict.items() if y == 2}

output:

{'c': 2, 'b': 2}

Answer 3

Well, you can use generator to produce found key values, one by one, instead of returning all of them at once.

The function search_keys returns generator

def search_keys(in_dict, query_val):
    return (key for key, val in in_dict.iteritems() if val == query_val)

# get keys, one by one
for found_key in search_keys(some_dict, 2):
    print(found_key)