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.y=y

I wonder how I can have the best implementation for finding the key that corresponds to a value. Something equivalent to:

def find_key(dic, val):
    for k,v in dic.items():
        if v==val:
            return k
    return None

NB: Sorry if this is obvious, I am a newbie in Python :-)

if you had googled it, you'd have found this too http://stackoverflow.com/questions/4504258/find-key-for-value-in-python-when-key-associated-with-multiple-values — samb8s, Jul 19 '11 at 13:55
If you intend to reverse mappings often, I would suggest you look at the following questions on SO: http://stackoverflow.com/questions/863935/a-data-structure-for-11-mappings-in-python and http://stackoverflow.com/questions/1063319/reversible-dictionary-for-python - I don't know enough python to say for sure that you can not do better than what you wrote in your question, but I would be very surprised. — Jean Hominal, Jul 19 '11 at 13:57
Sorry, I simplified the question, hence getting perfectly good answers that don't match my scenario: [I look for similar values](http://stackoverflow.com/questions/6749106/most-efficient-method-to-get-key-for-similar-values-in-a-dict) and not identical values — rds, Jul 19 '11 at 14:47

score 4 · Accepted Answer · answered Jul 19 '11 at 13:55

4

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())

answered Jul 19 '11 at 13:55

Daniel Roseman

Or `iteritems` if it is very large. – Fred Foo Jul 19 '11 at 13:57
this simply flops the dictionary - it doesn't explicitly return the key for a value, as the OP asked. – Nate Jul 19 '11 at 13:59
@Nate Well, I assumed they would then be able to work out how to look up the item in the new dictionary. – Daniel Roseman Jul 19 '11 at 14:13

score 1 · Answer 2 · answered Jul 19 '11 at 13:58

1

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]

answered Jul 19 '11 at 13:58

utdemir

score 0 · Answer 3 · answered Jul 19 '11 at 13:58

0

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)

answered Jul 19 '11 at 13:58

Nate

score 0 · Answer 4 · answered Jul 19 '11 at 21:24

0

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()])

answered Jul 19 '11 at 21:24

sampwing

4 Answers4