Lesson 7 — Frequency map

Counter: a frequency map in one line

Counting is so common that Python ships a ready-made tool: Counter from collections. Give it any sequence and it builds the key → count dict for you. .most_common(n) returns the n most frequent items as (item, count) pairs, biggest first.

from collections import Counter
Counter("banana")                  # {'a': 3, 'n': 2, 'b': 1}
Counter("banana").most_common(1)   # [('a', 3)]

Why it matters: "top 5 products", "most common error" — one line instead of a loop.