02·collections — Counter, defaultdict, deque

Introduction

The collections module extends built-in types. Counter counts occurrences and supports multiset arithmetic. defaultdict eliminates the "if key not in d" pattern — a factory creates the default value. deque is a double-ended queue with O(1) on both ends and optional maxlen. namedtuple provides an immutable tuple-like class with named fields. ChainMap merges multiple dicts into one view — ideal for layered configuration.