Source file found twice under different module names

[jaraco]

Starting with mypy 0.990, when testing a module in a namespace, mypy starts emitting errors when it encounters that module:

 error: Source file found twice under different module names: "abode" and "jaraco.abode"

The issue almost certainly has the same root cause as pytest-dev/pytest#3396. Pytest is too reliant on the old assumption that a directory without an __init__.py is not a package, but that assumption doesn't hold since Python 3.3.

[jaraco]

I observe that a similar error occurs when running the mypy manually:

 jaraco.abode main $ .tox/python/bin/mypy .
jaraco/abode/helpers/constants.py: error: Source file found twice under different module names: "abode.helpers.constants" and "jaraco.abode.helpers.constants"
jaraco/abode/helpers/constants.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
jaraco/abode/helpers/constants.py: note: Common resolutions include: a) adding `__init__.py` somewhere, b) using `--explicit-package-bases` or adjusting MYPYPATH
Found 1 error in 1 file (errors prevented further checking)

[jaraco]

It seems that mypy is giving a poor experience by default to namespace packages, and can be worked around by setting explicit package bases in the command line or config file.

[jaraco]

Closing in favor of python/mypy#14057.