python3 / mypy : how to manage stub files?

Question

I created a package "mypackage" that uses typing hints.

I import mypackage in a new module (not part of mypackage), mymodule, that also used typing hints.

I would like to apply mypy on mymodule, but get:

error: Cannot find implementation or library stub for module named 'mypackage'

I have been reading the documentation about stubs:

Yet, it is unclear to me what the best practices are.

Here something that works, but feels inefficient.

I create a package "mypackage". All the code is typed.
I call stubgen at the root of the package, it creates the folder ./out/mypackage with the stub files
I move this generated directory to a folder which has the stubs for my other packages, e.g. /path/to/stubs
I set the environment variable MYPYPATH to /path/to/stubs

mypy mymodule.py

now works, but is this the best way of doing things?

the information about the types is already in the source code, yet the information is duplicated in pyi files
we work as a group: everybody has to generate the stubs and copy them to their local stubs folder
everybody has to regenerate the stubs again when the code is updated

Most likely, I am missing something. Is there a simpler way to apply mypy to mymodule ?

I think posixsubprocess files are just the way to type annotate the code you don't want to touch. [This](https://stackoverflow.com/questions/59051631/what-is-the-use-of-stub-files-pyi-in-python) question has very nice answer on that topic — sudden_appearance, May 11 '22 at 12:46
@sudden_appearance I saw this answer, and it is one of the things which triggered my question. "You can also add type hints directly in the .py module like the following [...] But there are some cases where you want to keep them separate in stubs [...]". So it seems to indicate that if the type hints are in the .py source, you may not need to have to use .pyi files. But I could not figure out how to get things to work without the stubs. — Vince, May 11 '22 at 12:50
If you mean by _keeping separate_ that different parameter types result in different return types (which is done by overloading in stub files), then it can't be achieved without stub files (assuming `Union[]` or ` | ` are not applicable). But I think using stub files for proper type hint is overkill — sudden_appearance, May 11 '22 at 12:57
@sudden_appearance "using stub files for proper type hint is overkill" : is there any way not using stud files that would allow me to apply mypy on packages that import "mypackage"? I will edit my question to make it clearer — Vince, May 11 '22 at 15:57

score 1 · Answer 1 · answered May 11 '22 at 16:26

The error message in the question was displayed when using mypy version 0.761, but version 0.950 gives better information, as it points to:

which points in turn to:

It indicates that the setup.py of mypackage should include:

from distutils.core import setup

setup(
    # ...
    package_data={"mypackage": ["py.typed"]},
    #...
)

and an empty file py.typed should be placed in the mypackage folder.

After mypackage is (pip) installed, then applying mypy on mymodule works fine.

1 Answers1