what's the pythonic way to count the occurrence of an element in a list?

Question

this is what I did. is there a better way in python?

for k in a_list:   
  if kvMap.has_key(k):
    kvMap[k]=kvMap[k]+1   
  else:
    kvMap[k]=1

Thanks

Answer 1

Single element:

a_list.count(k)

All elements:

counts = dict((k, a_list.count(k)) for k in set(a_list))

Answer 2

I dunno, it basically looks fine to me. Your code is simple and easy to read which is an important part of what I consider pythonic.

You could trim it up a bit like so:

for k in a_list:
     kvMap[k] = 1 + kvMap.get(k,0)

Answer 3

Use defaultdict

from collections import defaultdict
kvmap= defaultdict(int)
for k in a_list:
    kvmap[k] += 1

Answer 4

Another solution exploits setdefault():

for k in a_list:
    kvMap[k] = kvMap.setdefault(k, 0) + 1

Answer 5

If your list is sorted, an alternative way would be to use itertools.groupby. This might not be the most effective way, but it's interesting nonetheless. It retuns a dict of item > count :

>>> import itertools
>>> l = [1,1,2,3,4,4,4,5,5,6,6,6,7]
>>> dict([(key, len([e for e in group]))
          for (key, group)
          in itertools.groupby(l)])
{1: 2, 2: 1, 3: 1, 4: 3, 5: 2, 6: 3, 7: 1}