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collections.defaultdict
Dict that auto-creates missing keys with a factory: `defaultdict(list)` returns `[]` for any new key. Replaces `if k not in d: d[k] = []`-style code in counters and grouping.
counts = defaultdict(int) counts[word] += 1
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Open lesson iv-43 →Related — Standard library
pathlib.PathObject-oriented filesystem paths. `Path(".").rglob("*.py")` beats `os.walk + os.…collections.CounterDict subclass that counts hashable items: `Counter("hello")` → `{'l': 2, ...}`.itertoolsIterator building blocks — `chain`, `groupby`, `combinations`, `accumulate`. Laz…functools.cacheMemoize a pure function with `@functools.cache`. Replaces hand-rolled dict cachi…dataclasses.fieldConfigure a dataclass attribute — mutable defaults, init/repr behavior. Use `fie…