132 – Efficiently count characters in a string

Python has a specialised dictionary for whenever you need to count things or represent counts: collections.Counter. The only restriction you need to keep in mind is that Counter can only count hashable objects, since it’s dictionary-like.

You can initialise by passing in an iterable of elements to count:

from collections import Counter

counts = Counter("mississippi")

From there, you can access counts by key, even if the key isn’t there:

print(counts["m"])  # 1
print(counts["z"])  # 0

Since it’s a specialised dictionary, Counter also includes many useful methods to work with counts, like the method most_common:

print(counts.most_common(2))
# [('i', 4), ('s', 4)]