What does Python's __missing__ actually do?

It only triggers on direct d[key] access, not get() or 'in'. Designed for mutation on miss.

Why doesn't defaultdict.get() use the default factory?

Same rule: get() avoids side effects. Use d[key] to invoke it.

Rarely. Prefer defaultdict or helpers. Overrides lead to subtle bugs.

Thought __missing__ had your back on missing dict keys? Think again. Python's design choices make custom dicts a minefield.

theAIcatchup Apr 08, 2026 3 min read

__missing__ ignores get() and __contains__ by design — no side-effect-free peeks. 𝕏
Overriding get() creates unreachable code and confusion; don't do it. 𝕏
Stick to setdefault or defaultdict for safe missing key handling. 𝕏

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Originally reported by dev.to