Best way to turn word list into frequency dict

Question

What's the best way to convert a list/tuple into a dict where the keys are the distinct values of the list and the values are the the frequencies of those distinct values?

In other words:

['a', 'b', 'b', 'a', 'b', 'c']
--> 
{'a': 2, 'b': 3, 'c': 1}

(I've had to do something like the above so many times, is there anything in the standard lib that does it for you?)

EDIT:

Jacob Gabrielson points out there is something coming in the standard lib for the 2.7/3.1 branch

Answer 1

Kind of

from collections import defaultdict
fq= defaultdict( int )
for w in words:
    fq[w] += 1

That usually works nicely.

Answer 2

I have to share an interesting but kind of ridiculous way of doing it that I just came up with:

>>> class myfreq(dict):
...     def __init__(self, arr):
...         for k in arr:
...             self[k] = 1
...     def __setitem__(self, k, v):
...         dict.__setitem__(self, k, self.get(k, 0) + v)
... 
>>> myfreq(['a', 'b', 'b', 'a', 'b', 'c'])
{'a': 2, 'c': 1, 'b': 3}

Answer 3

This is an abomination, but:

from itertools import groupby
dict((k, len(list(xs))) for k, xs in groupby(sorted(items)))

I can't think of a reason one would choose this method over S.Lott's, but if someone's going to point it out, it might as well be me. :)

Answer 4

I find that the easiest to understand (while might not be the most efficient) way is to do:

{i:words.count(i) for i in set(words)}

Answer 5

Just a note that, starting with Python 2.7/3.1, this functionality will be built in to the collections module, see this bug for more information. Here's the example from the release notes:

>>> from collections import Counter
>>> c=Counter()
>>> for letter in 'here is a sample of english text':
...   c[letter] += 1
...
>>> c
Counter({' ': 6, 'e': 5, 's': 3, 'a': 2, 'i': 2, 'h': 2,
'l': 2, 't': 2, 'g': 1, 'f': 1, 'm': 1, 'o': 1, 'n': 1,
'p': 1, 'r': 1, 'x': 1})
>>> c['e']
5
>>> c['z']
0