On Thu, Feb 11, 2021 at 12:00 AM Jim J. Jewett <jimjjew...@gmail.com> wrote:
> (1) Is it really a TypeGuard, or more of a TypeAssertion?
>
It's a query, not an assertion. The same concept is called type guard in
TypeScript.
> (2) Does this push too hard on "annotations only have one meaning"? If
> it has to imported from typing, then probably not, but I think that is
> worth adding to the PEP.
>
No. *This* particular annotation has a meaning for static type checkers. If
you use annotations for something else, don't use it.
> (3) Why can't the falsey case of an Optional String narrow to a set of
> literals {"", None} Are you worried that a subclass of str might have its
> own empty string, or just that you don't want to promise this? As written,
> it sounds like such a narrowing is forbidden.
>
A type checker could do this, but I don't see a reason why we should
prescribe that type checkers *should* do that. Some type checkers (in
particular, mypy) are reluctant to infer union types, and there are good
reasons for this (but this margin isn't wide enough to explain it).
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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