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Sarj Sarj Sarj Standards

prefer-match-assert-never

Typed enum dispatch must not silently ignore unhandled members.

SARJ032 Rule source Tests Catalog JSON

Why

A no-op wildcard or else branch hides missing enum members and lets newly added members pass unnoticed. Suppressing the argument-type error at assert_never also disables its static exhaustiveness guarantee.

Fix

Handle every enum member, bind the catch-all value, and pass it to typing.assert_never; raise an explicit exception when static exhaustiveness is unavailable. Narrow the accepted type or handle missing variants instead of suppressing assert_never's argument check.

Examples

Before — flagged Closed-set match silently ignores a variant
dispatch.py
from enum import StrEnum




class Kind(StrEnum):
    A = "a"
    B = "b"
    C = "c"




def handle(kind: Kind) -> None:
    match kind:
        case Kind.A:
            handle_a()
        case Kind.B:
            handle_b()
        case _:
            pass
After — preferred Closed-set match rejects an unhandled variant
dispatch.py
from enum import StrEnum
from typing import assert_never




class Kind(StrEnum):
    A = "a"
    B = "b"
    C = "c"




def handle(kind: Kind) -> None:
    match kind:
        case Kind.A:
            handle_a()
        case Kind.B:
            handle_b()
        case Kind.C:
            handle_c()
        case _ as unreachable:
            assert_never(unreachable)