How to know key in hash by value?

Question

I have hash:

test = {
    'a': ['localhost'],
    'b': ['bb','aa'],
    'c': ['cc']
}

For example I want to know key for bb ? (b).

this might be of assistance: http://stackoverflow.com/questions/3282823/python-get-key-with-the-least-value-from-a-dictionary — Nope, Mar 10 '11 at 14:52

score 4 · Accepted Answer · answered Mar 10 '11 at 14:55

4

In general you can construct a reverse dictionary like this:

test_reversed = dict((v, k) for k, values in test.iteritems() for v in values)

answered Mar 10 '11 at 14:55

Björn Pollex

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1

Not on python v3+... Would be `test.items()` rather than `test.iteritems()` – dawg Mar 11 '11 at 00:30

score 3 · Answer 2 · edited May 23 '17 at 10:33

There's not an easy way to do this other than iterating over the keys and values. If you need to do this a lot, it would be worth constructing the reversed mapping as a one off, so you can just look up that:

from collections import defaultdict

reversed_test = defaultdict(set)
test = { 'a': ['localhost'], 'b': ['bb','aa'], 'c': ['cc'] }

for k, v in test.items():
    for i in v:
        reversed_test[i].add(k)

print reversed_test['bb']

The reversed_test dictionary maps a key such as 'bb' to a set of strings that originally mapped to lists containing 'bb'. This is much less concise than Space_C0wb0y's neat solution since I was assuming it might be possible for test to look like:

{'a': ['foo','bar','foo'], 'b': ['foo','quux'] }

In other words:

multiple keys might map to lists containing 'foo'
'foo' might appear multiple times in a list.

score 2 · Answer 3 · answered Mar 10 '11 at 14:51

Assuming there is exactly one key matching, you can use

key = next(k for k in test if "bb" in test[k])

This iterates over all keys until the assigned list contains what you are looking for. This operation is much less efficient than the lookup by key, which is what dictionaries are made for.

How to know key in hash by value?

3 Answers3