Why?

pytest accepts parametrized values without checking them against the test’s annotations. This plugin lets mypy find incompatible values before the tests run. It supports multiple parameters, stacked decorators, and inline pytest.param calls.

Installation

pip install mypy-pytest-parametrize

Configure mypy to use the plugin

Add the plugin to your mypy configuration file:

pyproject.toml:

[tool.mypy]

plugins = [
    "mypy_pytest_parametrize",
]

mypy.ini, .mypy.ini, or setup.cfg:

[mypy]
plugins = mypy_pytest_parametrize

Karva

The same plugin configuration checks Karva tests using @karva.tags.parametrize. It supports Karva’s arg_names and arg_values keywords, stacked tags, and inline karva.param calls with IDs or tags. Karva must be installed in the environment where mypy runs.

For a single parameter, Karva treats each row as the parameter’s value even when the name is supplied as a list or tuple. For example, @karva.tags.parametrize(["value"], [(1, "x")]) supplies a tuple[int, str] to value.

Karva diagnostics use the existing pytest-parametrize error code.

Indirect fixtures and static limits

indirect=True is excluded because its values are fixture inputs, rather than the values received by the test. With indirect=["fixture_name"], the other parameters are still checked.

Parameter names and fixture routing determined at runtime are not checked. Stored decorator objects and class-level parametrization are also excluded. Stored pytest.param and karva.param objects and starred values within a row are also skipped when their individual types are unavailable. Values typed as Any retain mypy’s usual behavior.

The plugin follows the proposal in pytest issue #9334.