tallying occurrences in list

[kj]

Task: given a list, produce a tally of all the distinct items in
the list (for some suitable notion of "distinct").

Example: if the list is ['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b',
'c', 'a'], then the desired tally would look something like this:

[('a', 4), ('b', 3), ('c', 3)]

I find myself needing this simple operation so often that I wonder:

1. is there a standard name for it?
2. is there already a function to do it somewhere in the Python
standard library?

Granted, as long as the list consists only of items that can be
used as dictionary keys (and Python's equality test for hashkeys
agrees with the desired notion of "distinctness" for the tallying),
then the following does the job passably well:

def tally(c):
t = dict()
for x in c:
t[x] = t.get(x, 0) + 1
return sorted(t.items(), key=lambda x: (-x[1], x[0]))

But, of course, if a standard library solution exists it would be
preferable. Otherwise I either cut-and-paste the above every time
I need it, or I create a module just for it. (I don't like either
of these, though I suppose that the latter is much better than the
former.)

So anyway, I thought I'd ask. :)

~K

[Paul Rubin]

kj <no.e...@please.post> writes:
> 1. is there a standard name for it?

I don't know of one, or a stdlib for it, but it's pretty trivial.

> def tally(c):
> t = dict()
> for x in c:
> t[x] = t.get(x, 0) + 1
> return sorted(t.items(), key=lambda x: (-x[1], x[0]))

I like to use defaultdict and tuple unpacking for code like that:

from collections import defaultdict
def tally(c):
t = defaultdict(int)
for x in c:
t[x] += 1
return sorted(t.iteritems(), key=lambda (k,v): (-v, k))

[Peter Otten]

kj wrote:

Python 3.1 has, and 2.7 will have collections.Counter:

>>> from collections import Counter
>>> c = Counter("abcabcabca")
>>> c.most_common()
[('a', 4), ('c', 3), ('b', 3)]

Peter

[Magdoll]

On Jun 4, 11:28 am, Paul Rubin <no.em...@nospam.invalid> wrote:

I would also very much like to see this become part of the standard
library. Sure the code is easy to write but I use this incredibly
often and I've always wished I would have a one-line function call
that has the same output as the mysql query:

"SELECT id, count(*) FROM table GROUP BY somefield"

or maybe there is already a short solution to this that I'm not aware
of...

[Magdoll]

Thanks Peter, I think you just answered my post :)

[MRAB]

In Python 3 there's the 'Counter' class in the 'collections' module.
It'll also be in Python 2.7.

For earlier versions there's this:

http://code.activestate.com/recipes/576611/

[Lie Ryan]

If you're using previous versions (2.4 and onwards) then:

[(o, len(list(g))) for o, g in itertools.groupby(sorted(myList))]

[kj]

Thank you all!

~K

[Sreenivas Reddy Thatiparthy]

How about this one liner, if you prefer them;
set([(k,yourList.count(k)) for k in yourList])

[Paul Rubin]

Sreenivas Reddy Thatiparthy <thatiparth...@gmail.com> writes:
> How about this one liner, if you prefer them;
> set([(k,yourList.count(k)) for k in yourList])

That has a rather bad efficiency problem if the list is large.