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Make frozendict covariant in its value type - #16669
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Fixes #16668.
frozendictis immutable, so likeMappingit can be covariant in its value type. The key type stays invariant, since keys are still accepted as parameters (__getitem__,get), which matchesMapping[_KT, _VT_co].The class now uses a new
_VT_cotype variable.get(key, default: _VT_co)carries the same ignore commentsMapping.getuses for a covariant parameter. The__new__overloads keep their own_VT, which is solved per call.Added test cases: a
frozendict[str, int]is accepted wherefrozendict[str, int | None]is expected,get()still narrows as before, and the key type is still invariant.