When you do a relative import, __package__ is used to help resolve it, but if it isn't defined we fall back on using __name__ and __path__. We should probably consider raising an ImportWarning if __package__ isn't defined so that some day we can consider cleaning up __import__ and its call signature to be a bit more sane and drop the `globals` argument. It would also help people catch errors where they went overboard deleting attributes off a module.

We should probably even extend it to start using __spec__.parent and then falling back to __package__, and only after both are found missing do we raise the ImportWarning about needing to use __name__ and __path__. That way __import__ can simply start taking in the spec of the calling module to do all of its work instead of having to pass all of globals().

Patch with tests.  Not sure if importlib.h should be included?

Including importlib.h doesn't hurt.

that's what I figured.

Thanks for the quick review, new patch uploaded.

New changeset 6908b2c9a404 by Brett Cannon in branch 'default':
Issue #25791: Raise an ImportWarning when __spec__ or __package__ are
https://hg.python.org/cpython/rev/6908b2c9a404

Thanks for the patch, Rose! I did notice your review comments about the missing goto and the stacklevel and I simply added them myself.

Favouring __spec__.parent over __package__ will break the documented workaround in PEP 366 for enabling explicit relative imports from __main__ even when a module is run directly instead of via -m:

    if __name__ == "__main__" and __package__ is None:
        __package__ = "expected.package.name"

It may be that workaround is something we *want* to break (as per http://bugs.python.org/issue21762#msg258332 ), but if so, it's at least worthy of a porting note in the What's New document.

As I commented on another issue, I think I'm going to tweak the change to continue to prefer __package__, but raise ImportWarning when it doesn't match __spec__.parent. Once we do that for all attributes we can wait until Python 2.7 is done and then swap priority to __spec__ and raise a DeprecationWarning if __spec__ is missing. That way post-Python 2.7 we can move entirely to a spec-based import system.

New changeset 219c44fe8968 by Brett Cannon in branch 'default':
Issue #25791: Warn when __package__ != __spec__.parent.
https://hg.python.org/cpython/rev/219c44fe8968