Untyped decorator makes function "foo" untyped

[adamjstewart]

Bug Report

Our CI tests started failing with the mypy 0.920 release. It looks like a new feature is checking decorator types? This may not be a bug in mypy but a lack of documentation explaining the new feature. I couldn't find any release notes or CHANGELOG explaining this feature.

To Reproduce

1. Create a new file:

import pytest

@pytest.fixture
def foo() -> None:
    pass

2. Run mypy: mypy --strict --ignore-missing-imports foo.py

Expected Behavior

Since the function is correctly typed, I wouldn't expect mypy to complain, but there must be a new feature triggering this. Is it because the pytest library is untyped? Normally mypy skips those kind of things.

Actual Behavior

Mypy complains with an error message I can't find in the documentation:

foo.py:3: error: Untyped decorator makes function "foo" untyped

Your Environment

• Mypy version used: 0.920
• Mypy command-line flags: --strict --ignore-missing-imports
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.9.9
• Operating system and version: macOS 10.15.7

@calebrob6

[nth10sd]

What version of pytest are you using? pytest version 6.0.0rc1 onwards has type information.

[hauntsaninja]

I'm not aware of any changes in mypy 0.920 that would affect this. As Gary says, make sure you're using pytest>=6 and that it is installed in the same virtual environment as mypy (or that you use the --python-executable flag to point mypy to the python which has pytest installed)

[srittau]

Just as a point of reference: We also started getting decorator-related mypy errors with mypy 0.920:

_R = TypeVar("_R")
_P = ParamSpec("_P")

def transaction(f: Callable[Concatenate[Transaction, _P], _R]) -> Callable[_P, _R]: ...
@transaction
def foo(): ...
x = foo()  # Need type annotation for "x"  [var-annotated]

This didn't trigger an error before, although ParamSpec isn't properly supported. For now we changed the definition to the following to work around this:

def transaction(f: Callable[..., _R]) -> Callable[..., _R]: ...

[adamjstewart]

I'm using pytest 6.2.5 (the latest version).

[adamjstewart]

Based on @srittau's comment, is it safe to assume that this is a bug in mypy and not a lack of typing support in pytest? It seems like this error is raised even when the decorator is typed and no external libraries are used.

[pranavrajpal]

What happens when you run mypy without --ignore-missing-imports? The only way I've been able to reproduce this is if pytest is not installed at all, which causes pytest.fixture to be silently turned into Any due to the --ignore-missing-imports flag, leading to the error message about an untyped decorator.

Based on @srittau's comment, is it safe to assume that this is a bug in mypy

That problem seems unrelated to your issue. Those errors are probably due to incomplete ParamSpec support, but pytest's type for fixture here doesn't seem to use ParamSpec.

[adamjstewart]

What happens when you run mypy without --ignore-missing-imports?

I get an error saying it can't find type hints:

$ mypy --strict test.py 
test.py:1: error: Cannot find implementation or library stub for module named "pytest"
test.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
test.py:3: error: Untyped decorator makes function "foo" untyped
Found 2 errors in 1 file (checked 1 source file)

However, pytest is installed and in both my PATH and PYTHONPATH:

$ which pytest
/Users/Adam/.spack/.spack-env/view/bin/pytest
$ python
Python 3.9.9 (main, Dec 15 2021, 16:29:14) 
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pytest
>>> pytest.__file__
'/Users/Adam/.spack/.spack-env/view/lib/python3.8/site-packages/pytest/__init__.py'

If I add --python-executable=$(which python) it doesn't seem to make a difference. I've reproduced this with both a Spack environment locally and in GitHub Actions CI (I'm assuming setup-python uses pip and virtualenv?). How does mypy find type hints?

[hauntsaninja]

What is the result of:

python -m pip show pytest
python -m pip show mypy
python -m mypy -c 'import pytest; reveal_type(pytest.fixture)'

[adamjstewart]

$ python -m pip show pytest
Name: pytest
Version: 6.2.4
Summary: pytest: simple powerful testing with Python
Home-page: https://docs.pytest.org/en/latest/
Author: Holger Krekel, Bruno Oliveira, Ronny Pfannschmidt, Floris Bruynooghe, Brianna Laugher, Florian Bruhin and others
Author-email: None
License: MIT
Location: /Users/Adam/.spack/.spack-env/._view/tmizjnr2yebouizawyxzmo3gwignw5lz/lib/python3.8/site-packages
Requires: attrs, iniconfig, packaging, pluggy, py, toml
Required-by: pytest-mock, pytest-cov, nbmake

$ python -m pip show mypy
Name: mypy
Version: 0.920
Summary: Optional static typing for Python
Home-page: http://www.mypy-lang.org/
Author: Jukka Lehtosalo
Author-email: jukka.lehtosalo@iki.fi
License: MIT License
Location: /Users/Adam/spack/opt/spack/darwin-catalina-ivybridge/apple-clang-12.0.0/py-mypy-0.920-fimyp26g6xfazrl4ibu5mvkwlwd33egu/lib/python3.9/site-packages
Requires: typing-extensions, mypy-extensions, tomli
Required-by: 

$ python -m mypy -c 'import pytest; reveal_type(pytest.fixture)'
<string>:1: note: Revealed type is "Any"

Note that this is on my local installation which uses Spack, not pip.

[hauntsaninja]

I don't know anything about Spack, is it doing something weird to python? E.g., it looks like pytest is installed on Python 3.8 and mypy is installed on Python 3.9. So this definitely would not work without --python-executable.

If python is something unusual or shimmed, I'm not sure --python-executable will work.

[adamjstewart]

Ah yes, Spack does many weird things. All packages are installed to a separate installation prefix allowing you to install multiple versions of the same package with different compilers. I just tried again with only Python 3.9 versions of mypy and pytest in my PYTHONPATH and I'm still getting the same error. Let me try on another system using conda/pip instead of Spack.

[adamjstewart]

This seems to work inside a Conda environment:

$ python -m mypy -c 'import pytest; reveal_type(pytest.fixture)'
<string>:1: note: Revealed type is "Overload(def [_FixtureFunction <: def (*Any, **Any) -> builtins.object] (fixture_function: _FixtureFunction`-1, *, scope: Union[Union[Literal['session'], Literal['package'], Literal['module'], Literal['class'], Literal['function']], def (builtins.str, _pytest.config.Config) -> Union[Literal['session'], Literal['package'], Literal['module'], Literal['class'], Literal['function']]] =, params: Union[typing.Iterable[builtins.object], None] =, autouse: builtins.bool =, ids: Union[typing.Iterable[Union[None, builtins.str, builtins.float, builtins.int, builtins.bool]], def (Any) -> Union[builtins.object, None], None] =, name: Union[builtins.str, None] =) -> _FixtureFunction`-1, def (fixture_function: None =, *, scope: Union[Union[Literal['session'], Literal['package'], Literal['module'], Literal['class'], Literal['function']], def (builtins.str, _pytest.config.Config) -> Union[Literal['session'], Literal['package'], Literal['module'], Literal['class'], Literal['function']]] =, params: Union[typing.Iterable[builtins.object], None] =, autouse: builtins.bool =, ids: Union[typing.Iterable[Union[None, builtins.str, builtins.float, builtins.int, builtins.bool]], def (Any) -> Union[builtins.object, None], None] =, name: Union[builtins.str, None] =) -> _pytest.fixtures.FixtureFunctionMarker)"

Which raises the question of why it doesn't work with Spack or with GitHub Actions. The latter doesn't use Spack at all. Where does mypy search for type hints?

[adamjstewart]

Where does mypy search for type hints?

I believe the answer to this question is https://mypy.readthedocs.io/en/stable/running_mypy.html#finding-imports

I'm guessing something gets found before the actual pytest .py files and that file is missing type hints. Does mypy have a way to print which step of the search process is being used?