(1) Is it really a TypeGuard, or more of a TypeAssertion?
(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.
(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.
-jJ
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