Most efficient method to get key for a value in a dict

Question

I have a dictionary of objects:

dic = {\'k1\':obj1, \'k2\':obj2, \'k3\':obj3, ...}

class MyObject:
    def __init__(self,x,y):
        self.x=x
        self         


        

Answer 1

Ignoring the possibility that samb8s raises, that multiple keys exist for a single value, you can easily reverse the dictionary like this:

reverse_dic = dict((v, k) for k, v in dic.items())

Answer 2

This will return a set of ALL keys that have that value - like a filter by value:

def find_keys(dic, val):
    return set(k for k,v in dic.items() if v == val)

Answer 3

regular dict[key] -> value

my_regular_dict = {1: 2, 3: 4, 5: 6}

reverse lookup, dict[value] -> key

my_reverse_lookup = dict([(val, key) for key, val in my_regular_dict.items()])

Answer 4

You can use list comprehension. This code gives you the keys in a list:

>>> a
{1: 2, 3: 4, 9: 4, 5: 6, 7: 8}
>>> [key for key, value in a.items() if value == 4]
[3, 9]