Return Key By Value In Dictionary

I am trying to return the key in a dictionary given a value in this case if 'b' is in the dictionary, I want it to return the key at which 'b' is (i.e 2) def find_key(input_dict,

Solution 1:

Return first matching key:

deffind_key(input_dict, value):
    returnnext((k for k, v in input_dict.items() if v == value), None)

Return all matching keys as a set:

def find_key(input_dict, value):
    return {k for k, v in input_dict.items() ifv== value}

Values in a dictionary are not necessarily unique. The first option returns None if there is no match, the second returns an empty set for that case.

Since the order of dictionaries is arbitrary (dependent on what keys were used and the insertion and deletion history), what is considered the 'first' key is arbitrary too.

Demo:

>>>deffind_key(input_dict, value):...returnnext((k for k, v in input_dict.items() if v == value), None)...>>>find_key({1:'a', 2:'b', 3:'c', 4:'d'}, 'b')
2
>>>find_key({1:'a', 2:'b', 3:'c', 4:'d'}, 'z') isNone
True
>>>deffind_key(input_dict, value):...return {k for k, v in input_dict.items() if v == value}...>>>find_key({1:'a', 2:'b', 3:'c', 4:'d'}, 'b')
set([2])
>>>find_key({1:'a', 2:'b', 3:'c', 4:'d', 5:'b'}, 'b')
set([2, 5])
>>>find_key({1:'a', 2:'b', 3:'c', 4:'d'}, 'z')
set([])

Note that we need to loop over the values each time we need to search for matching keys. This is not the most efficient way to go about this, especially if you need to match values to keys often. In that case, create a reverse index:

from collections import defaultdict

values_to_keys = defaultdict(set)

forkey, value in input_dict:
    values_to_keys[value].add(key)

Now you can ask for the set of keys directly in O(1) (constant) time:

keys = values_to_keys.get(value)

This uses sets; the dictionary has no ordering so either, sets make a little more sense here.

Solution 2:

Amend your function as such:

def find_key_for(input_dict, value):    
    for k, v in input_dict.items():
        if v == value:
            yield k

Then to get the first key (or None if not present)

printnext(find_key_for(your_dict, 'b'), None)

To get all positions:

keys = list(find_key_for(your_dict, 'b'))

Or, to get 'n' many keys:

from itertools importislicekeys= list(islice(find_key_for(your_dict, 'b'), 5))

Note - the keys you get will be 'n' many in the order the dictionary is iterated.

If you're doing this more often than not (and your values are hashable), then you may wish to transpose your dict

from collections import defaultdict

dd = defaultdict(list)
for k, v in d.items():
    dd[v].append(k)

printdd['b']