return key by value in dictionary [duplicate]

Answer 1

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)

print next(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 import islice
keys = 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)

print dd['b']