Question

Let’s say I have a Counter object: Counter({'test': 2, 'this': 1, 'is': 1})

I would like to iterate over this object in the following way:

c = Counter({'test': 2, 'this': 1, 'is': 1})

for i,s in my_counter_iterator(c):
    print i, ":", s

>> 1 : ['this', 'is']
>> 2 : ['test']

How do I do that efficiently (this code should run on a web server for each request…)?

EDIT

I have tried this, but I have the feeling there are more efficient ways. Are there?

from itertools import groupby

for k,g in groupby(sorted(c.keys(), key=lambda x: c[x]),key=lambda x: c[x]):
    print k, list(g)


1 ['this', 'is']
2 ['test']

Answer 1

If you want to do this with large Counters, you really have no choice but to invert the mapping.

inv_c = defaultdict(list)
for k, v in c.iteritems():
    inv_c[v].append(k)

Then inv_c.iteritems() is what you want.