match_exhaustiveness error

Non-exhaustive match statement

A value-dispatch match statement that has no irrefutable branch may fail to handle certain runtime values, leading to a silent fall-through (Python does not raise an error for unmatched match subjects). Basilisk reports this as an error.

Two cases are not flagged, matching the reference checkers: a bare capture case name: (no guard) is irrefutable — like case _:, it makes the match exhaustive; a structural match (sequence/mapping patterns) decomposes open-ended shapes — e.g. narrowing a tuple union of mixed arity — where a catch-all is not required for correctness.

How to handle it

Every rule is on by default — strict is the default, not a cage. You can dial match_exhaustiveness down per-file or per-path from your editor or pyproject.toml, or fix the code so it type-checks. See the Type System rules and the complete diagnostic reference.

Canonical URL: https://www.basilisk-python.dev/errors/match_exhaustiveness