PEP 827: Type Manipulation

[steve.dower]

There is a demo of a runtime evaluator, which is also where this PEP draft currently lives.

There is an in-progress proof-of-concept implementation in mypy.

This seems like something that could be published first as an extension to a type checker (presumably mypy) and gain some real world use before it gets committed into a standard?

def select[ModelT, K: typing.BaseTypedDict](
    typ: type[ModelT],
    /,
    **kwargs: Unpack[K],
) -> list[
    typing.NewProtocol[
        *[
            typing.Member[
                c.name,
                ConvertField[typing.GetMemberType[ModelT, c.name]],
            ]
            for c in typing.Iter[typing.Attrs[K]]
        ]
    ]
]:
    raise NotImplementedError

Not going to lie, this is not a compelling example to me. I don’t want to see this in code, scroll past it, or implement a static evaluator in a type checker to calculate the result (that last one is probably the most valid feedback, and yes, my job used to involve implementing static evaluators in type analysis engines for Python ).

Perhaps what we really need is a better extension model for type checkers, so that libraries that need this level of complexity can provide something separate from their code but still in the repository/package and accessible?