Mypy won't follow imports to local files. " cannot find implementation"

[Luttik]

Bug Report

The exception "error: Cannot find implementation or library stub for module named "mypy_src_issue_example.a" is thrown for local files that are in scope, even when py.typing is added to the project. An example project is added here: mypy-local-import-issue-example, but the gist is that:

For a file src/my_project/a.py

def foo() -> str:
    ...

and a file src/my_project/a.py

from my_project.a import foo

foo()

The following exception will be thrown:

my_project\b.py:1: error: Cannot find implementation or library stub for module named "my_project.a"
src\my_project\b.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 2 source files)

this happens with and without the following pyproject.toml config:

# more non-mypy config above

[tool.mypy]
mypy_path = "src"
python_version = "3.8"

To Reproduce

I've stored a small examplary project here: mypy-local-import-issue-example

Expected Behavior
It should not throw any error since there is no reason why it shouldn't be able to resolve src/my_project/a.py.

Your Environment

• Mypy version used: "0.910"
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): see above
• Python version used: 3.8
• Operating system and version: Windows 10 Pro (build 19043.1165)

[hauntsaninja]

I believe this should work if you set --namespace-packages=True (this should be made the default, see #8584)

[pietermarsman]

An alternative solution is to add a __init__.py file to the directory. This indicates that the directory is a python module.

[KomaGR]

An alternative solution is to add a __init__.py file to the directory. This indicates that the directory is a python module.

Why might this solution not be working for me? I'm not entirely familiar with python modules but this didn't work whereas adding --namespace-packages=True worked.