Counter

The Counter counts objects passed to it. Sometimes it's very convenient to just dump some list into the counter and immediately get a data structure with counted elements.

from collections import Counter

shirts_colors = ["red", "white", "blue", "white", "white", "black", "black"]
c = Counter(shirts_colors)
print(c)

c["blue"] += 1
print(f"After shopping: {c}")

Counter({'white': 3, 'black': 2, 'red': 1, 'blue': 1})
After shopping: Counter({'white': 3, 'blue': 2, 'black': 2, 'red': 1})

The following code explains how Counter() works using defaultdict():

from collections import defaultdict

shirts_colors = ["red", "white", "blue", "white", "white", "black", "black"]

d = defaultdict(int)
for shirt in shirts_colors:
    d[shirt] += 1

print(d)

defaultdict(<class 'int'>, {'red': 1, 'white': 3, 'blue': 1, 'black': 2})