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Python: finding keys with unique values in a dictionary?

Asked 2009-06-23T12:35:47.520
18

I receive a dictionary as input, and want to return a list of keys for which the dictionary values are unique in the scope of that dictionary.

I will clarify with an example. Say my input is dictionary a, constructed as follows:

a = dict()
a['cat'] =      1
a['fish'] =     1
a['dog'] =      2  # <-- unique
a['bat'] =      3
a['aardvark'] = 3
a['snake'] =    4  # <-- unique
a['wallaby'] =  5
a['badger'] =   5

The result I expect is ['dog', 'snake'].

There are obvious brute force ways to achieve this, however I wondered if there's a neat Pythonian way to get the job done.

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2 Answers

15

I think efficient way if dict is too large would be

countMap = {}
for v in a.itervalues():
    countMap[v] = countMap.get(v,0) + 1
uni = [ k for k, v in a.iteritems() if countMap[v] == 1]
answered 2009-06-23T13:02:44.970
4
>>> b = []
>>> import collections
>>> bag = collections.defaultdict(lambda: 0)
>>> for v in a.itervalues():
...     bag[v] += 1
...
>>> b = [k for (k, v) in a.iteritems() if bag[v] == 1]
>>> b.sort() # optional
>>> print b
['dog', 'snake']
>>>
answered 2009-06-23T13:10:45.813

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