Thanks Eric for the PEP. I have some questions: *Type checkers should assume that type narrowing should be applied to the expression that is passed as the first argument to a user-defined type guard.*What should happen in case of instance methods, class methods or static methods? The first two cases have an implicit self/class argument? Would the type guyard apply to that argument or to the first explicit parameter? What about properties? Would they be supported? *User-defined type guards apply narrowing only in the positive case (the if clause).*Like Sebastian Rittau mentioned, what is the rationale behind this? On Wed, Oct 21, 2020 at 2:41 PM Jake Bailey via Typing-sig < typing-sig@python.org> wrote:
We talked about this on gitter, but I'll sum it up here as well. I think `filter` could be modified to use a guard:
``` @overload def filter(__function: Callable[[_T1], TypeGuard[_T2]], __iterable: Iterable[_T1]) -> Iterator[_T2]: ... ```
Unfortunately, you have to encode the narrowing behavior into the signature (as there's no code flow graph), but I suppose this makes some sense.
`filterfalse`, not so much, because the way `TypeGuard` has to work only allows proving of truths. You mentioned TS's `Exclude`, which would allow proving of truths by having a `TypeGuard` that could say something like "I prove that the type of the first parameter is not of type `_T2`", for example. _______________________________________________ Typing-sig mailing list -- typing-sig@python.org To unsubscribe send an email to typing-sig-leave@python.org https://mail.python.org/mailman3/lists/typing-sig.python.org/ Member address: skreft@gmail.com
-- Sebastian Kreft