Counter, defaultdict, namedtuple

Why this matters

Counter, defaultdict, and deque are the difference between clean code and reinventing wheels in interviews and production. deque is O(1) at both ends — list.pop(0) is O(n), a bug that quietly slows queue workers.

Common pitfalls

• Using dict.get(k, []).append(v) instead of defaultdict(list) — the .get version never mutates the dict.
• Using list as a FIFO queue — pop(0) is O(n); reach for collections.deque for O(1) popleft.
• Sorting a Counter manually when .most_common(n) already returns the top-n by count.