defaultdict

defaultdict is a dict-like container from Python's collections module that provides a default value for missing keys via a factory function passed at construction time. If a key is accessed that is not present, defaultdict automatically creates a value by calling the default_factory and inserts the key with that value, returning it. This behavior contrasts with a standard dict, which raises KeyError for missing keys. If default_factory is None, defaultdict behaves like a regular dict.

Common factories include int (for counting, because int() returns 0), list (to collect items for each key), and set (to accumulate unique items). The default_factory can be changed after creation.

Usage examples: d = defaultdict(list); d['a'].append(1) results in {'a': [1]} without KeyError. For counting occurrences, d = defaultdict(int); for x in data: d[x] += 1.

Compared to pure dict, defaultdict reduces code for missing-key handling, especially in grouping and accumulation tasks. It is not appropriate when a missing key should be allowed to raise an exception or when you need explicit KeyError semantics; in such cases, use a normal dict or handle missing keys differently. It is often used in data processing tasks, such as grouping, bucketing, or building inverted indices. It is part of the standard library and coexists with other collections like Counter.