Python-сообщество

>>> slovo = "aaasfvcgh"
>>> { k:slovo.count(k) for k in set(list(slovo)) }
{'a': 3, 'c': 1, 'g': 1, 'f': 1, 'h': 1, 's': 1, 'v': 1}

>>> from collections import Counter
>>> Counter('vasya')
Counter({'a': 2, 'y': 1, 's': 1, 'v': 1})

>>> d = {}
>>> slovo = "aaasfvcgh"
>>> for x in set(slovo):
...     d.update({x: slovo.count(x)})
... 
>>> print d
{'a': 3, 'c': 1, 'g': 1, 'f': 1, 'h': 1, 'v': 1, 's': 1}

class Counter(dict):
    def __init__(self, iterable=None, **kwds):
        super(Counter, self).__init__()
        self.update(iterable, **kwds)
    def update(self, iterable=None, **kwds):
        if iterable is not None:
            if isinstance(iterable, Mapping):
                if self:
                    self_get = self.get
                    for elem, count in iterable.iteritems():
                        self[elem] = self_get(elem, 0) + count
                else:
                    super(Counter, self).update(iterable)
            else:
                self_get = self.get
                for elem in iterable:
                    self[elem] = self_get(elem, 0) + 1
        if kwds:
            self.update(kwds)

[guest@localhost ~]$ python3 -mtimeit -s "import collections; s = 'abc' * 100" "collections.Counter(s)"
10000 loops, best of 3: 173 usec per loop
[guest@localhost ~]$ 
[guest@localhost ~]$ python3 -mtimeit -s "s = 'abc' * 100" "{k: s.count(k) for k in set(s)}"
100000 loops, best of 3: 13.8 usec per loop
[guest@localhost ~]$