`pre-commit run mypy` has a different behavior than `mypy` regarding giving preference to `.pyi` files over `.py`

I’ve created a minimal working example here: https://github.com/tadeu/pre-commit-mypy-issue

It’s about this exact situation described in mypy documentation:

If a directory contains both a .py and a .pyi file for the same module, the .pyi file takes precedence. This way you can easily add annotations for a module even if you don’t want to modify the source code. This can be useful, for example, if you use 3rd party open source libraries in your program (and there are no stubs in typeshed yet).

The problem is that mirrors-mypy is passing all files explicitly in the command line, and it seems to be passing the .py files instead of the .pyi files in this case:

$ pre-commit run mypy -a
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

src/example/__init__.py:6: error: Function is missing a type annotation  [no-untyped-def]
src/example/something.py:5: error: Call to untyped function "Example" in typed context  [no-untyped-call]
Found 2 errors in 2 files (checked 2 source files)

[sziem]

I ran into this issue today after adding a mypy-protobuf stub file to my code.

I ended up using @max-sixty 's suggestion for the file pattern plus an explicit exclude of the file duplicated by the stub file, so that only the stub file is used for type-checking by mypy.

  - repo: local
    hooks:
      - id: mypy-local
        name: mypy
        entry: mypy
        language: python
        types: [python]
        files: '.*\.py[i]?'
        exclude: '/path/to/file_that_exists_as_both_py_and_pyi.py'
        # args: ["--ignore-missing-imports", "--scripts-are-modules"] (add if you use them)
        require_serial: true

This is feasible, if the number of stub files is small and not expected to change. If the number is large, I guess we’d need some kind of pattern to express that the pyi file is always to be preferred over the py file. An ugly workaround could be to give all files with stubs a common suffix, so that they can be excluded with a pattern like .*_suffix.py

[s-weigand]

After just adding stubs I had the same issue. My wasteful but working workaround is to just run mypy on the whole source folder, as I would from the command line. By using pass_filenames: false you prevent pre-commit from passing the staged files to mypy. Hope this helps someone 😄

  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v0.790
    hooks:
      - id: mypy
        exclude: ^docs
        args: ["--ignore-missing-imports", "--scripts-are-modules", <project_source_dir>]
        pass_filenames: false

My guess is that this issue boils down to a difference between how mypy processes folders (where it possibly swaps *.py files for *.pyi files) and file paths (possibly not filtering and/or looking up) as arguments.

@tadeu could you please link the issue at mypy so I can follow it?