Counter supports three forms of initialization. Its constructor can be called with a sequence of items, a dictionary containing keys and counts, or using keyword arguments mapping string names to counts.

import collections

print collections.Counter(['a', 'b', 'c', 'a', 'b', 'b'])
print collections.Counter({'a':2, 'b':3, 'c':1})
print collections.Counter(a=2, b=3, c=1)

The results of all three forms of initialization are the same.

$ python

Counter({'b': 3, 'a': 2, 'c': 1})
Counter({'b': 3, 'a': 2, 'c': 1})
Counter({'b': 3, 'a': 2, 'c': 1})

Use most_common() to produce a sequence of the n most frequently encountered input values and their respective counts.

import collections

c = collections.Counter()
with open('/usr/share/dict/words', 'rt') as f:
    for line in f:

print 'Most common:'
for letter, count in c.most_common(3):
    print '%s: %7d' % (letter, count)

This example counts the letters appearing in all of the words in the system dictionary to produce a frequency distribution, then prints the three most common letters.

$ python

Most common:
e:  235331
i:  201032
a:  199554