Do not export imported names in stubs unless using 'as'

[JukkaL]

If a stub for module x imports something from typing, for example, currently from x import * also imports that name, which is wrong, since generally the module implementation does not import anything from typing, and besides it's just a bad idea in general.

However, we don't want to define __all__ for every stub, as that would be painful and redundant, since stubs mostly contain only exported features. Maybe we should not export any imported names by default, unless the import uses as. For example:

from typing import List    # Not exported
import os.path as path   # Exported
import os as os   # Also exported

This would only affect stubs. We need a way of knowing which files are stubs. Initially, just anything that is under stubs/ should be considered a stub.

[ambv]

Sorry, I disagree here. import os as os is unexpected. __all__ is verbose but at least people expect and understand it (hopefully?). Also, the fact that as was used is unavailable during runtime which might or might not be relevant in the future, for instance when somebody tries to implement runtime checks.

[gvanrossum]

See my counter-proposal regarding __all__ in python/typing#22 (comment)

[JukkaL]

We'll use the approach linked to in Guido's comment instead.

[JukkaL]

Update: We are back to using as: python/typing@4215e09

Also, now we can detect stubs easily as they use the .pyi extension, so this should be straightforward to implement.