error: No parent module -- cannot perform relative import

[renyuneyun]

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

A bug.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

I have PYTHONPATH=$HOME/self/exp, and I'm working on a library under $HOME/self/exp/exp. The directory structure looks like this:

exp
├── exp
     ├── __init__.py
     ├── a.py
     ├── b.py

Sample minimal code is under https://gist.github.com/renyuneyun/0ff13a8c9013f178c760f963b5eba502 . Basically I use relative imports to import another file under the same module.
__init__.py is empty.

• What is the actual behavior/output?

a.py:4: error: No parent module -- cannot perform relative import

• What is the behavior/output you expect?

Pass with no errors or warnings

• What are the versions of mypy and Python you are using?

mypy 0.670
Python 3.7.3

• Do you see the same issue after installing mypy from Git master?

Yes. No difference.

• What are the mypy flags you are using? (For example --strict-optional)

No flags.

[gvanrossum]

Is the current directory when you're invoking mypy perhaps exp/exp? It should be the parent thereof.

[renyuneyun]

Thanks a lot. That did make the error disappear.
But... why doesn't the original way work?

[gvanrossum]

This is the same issue as #6660. Closing this as a duplicate.