Replace implicit reexport with explicit

[aganders3]

Feature

I would like to create a way to explicitly suppress a specific implicit export.

Pitch

I am working with a wrapper library that attempts to unify an interface between multiple backends. So for example it does something like this:

# my_lib/foo.py

from . import BACKEND_A, BACKEND_B

if BACKEND_A:
    from lib_a.foo import *

    Confabulator.bar = Confabulator.baz

elif BACKEND_B:
    from lib_b.foo import *

The implicit export via star-import here is helpful because there are a lot of classes to import, more may be added, and most of them don't need to be modified like this at all.

Unfortunately this causes some minor problems with typing - I get an attr-defined error depending on the installed backend:

from my_lib.foo import Confabulator
c = Confabulator()
reveal_type(c)  # revealed type is lib_a.foo.Confabulator
c.bar()  # raises an [attr-defined] error

At first I thought this was an issue with adding class attributes after definition (rejected in #5363), but it's also a problem if I try to instead to subclass and re-assign. I think the implicit re-export is getting in the way of redefining the class.

My initial thought on how to achieve this would be to allow from lib_a.foo import Confabulator as _Confabulator after the from lib_a.foo import * to kind of undo the implicit export. I poked around in the code a bit but have not figured out the best way to do this yet. I do worry this would be a breaking change, however. I am open to any other suggestions or workarounds!