[Python-ideas] PEP 563 and expensive backwards compatibility

[Jim J. Jewett]

I am generally supportive of leaving the type annotations unprocessed
by default, but there are use cases where they should be processed
(and even cases where doing it at the right time matters, because of a
side effect).

I am concerned that the backwards compatibility story for non-typing
cases be not just possible, but reasonable.

(1)  The PEP suggests opting out with @typing.no_type_hints ... The
closest I could find was @typing.no_type_check, which has to be called
on each object.

It should be possible to opt out for an entire module, and it should
be possible to do so *without* first importing typing.

Telling type checkers to ignore scopes (including modules) with a

# typing.no_type_hints

comment would be sufficient for me.

If that isn't possible, please at least create a nontyping or
minityping module so that the marker can be imported without the full
overhead of the typing module.

(2)  Getting the annotations processed (preferably at the currently
correct time) should also be possible on a module-wide basis, and
should also not require importing the entire typing apparatus.

It would be a bit messy (like the old coding cookie), but recognizing
a module-wide

# typing.no_type_hints

comment and then falling back to the current behavior would be enough for me.

Alternatively, it would be acceptable to run something like
typing.get_type_hints, if that could be done in a single pass at the
end of the module (callable from both within the module and from
outside) ... but again, such a cleanup function should be in a smaller
module that doesn't require loading all of typing.

-jJ