if you need to count characters in string, try the following
a = "aaabbcccd"
b = dict.fromkeys(a, 0)
for i in a:
b[i] += 1
b now holds the counts you require:
{'a': 3, 'c': 3, 'b': 2, 'd': 1}
solved Counting the occurrence of unique letters in a string in Python 3