mypy plugin for @mark.parametrize

[Jasha10]

The idea is that a plugin would allow the arguments to the @pytest.mark.parametrize decorator to be type-checked.

For example:

import pytest

@pytest.mark.parametrize(
    "arg",
    [
        123,        # correct type int
        456.789,    # wrong type float, mypy plugin to complain
    ]
)
def test_foo(
    arg: int   # plugin reads `int` annotation here
) -> None:
    ...

[The-Compiler]

That seems like a good idea - but I don't think it should be part of pytest core.

[tirkarthi]

Nice idea. I found that there is typeguard project which can be used to perform run-time type checks.

# conftest.py
import typeguard


def pytest_collection_modifyitems(session, config, items):
    for item in items:
        item.obj = typeguard.typechecked(item.obj)

pytest -vvv          
==================================== test session starts ====================================
platform linux -- Python 3.10.1, pytest-6.2.4, py-1.10.0, pluggy-0.13.1 -- /home/karthikeyan/stuff/python/myenv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/testdir1
plugins: forked-1.4.0, typeguard-2.13.3, anyio-3.5.0, mypy-plugins-1.9.3, xdist-2.5.0
collected 2 items                                                                           

tests/test_foo.py::test_foo[123] PASSED                                               [ 50%]
tests/test_foo.py::test_foo[456.789] FAILED                                           [100%]

========================================= FAILURES ==========================================
_____________________________________ test_foo[456.789] _____________________________________

args = (), kwargs = {'arg': 456.789}, memo = <typeguard._CallMemo object at 0x7fd9fc7d2620>

    def wrapper(*args, **kwargs):
        memo = _CallMemo(python_func, _localns, args=args, kwargs=kwargs)
>       check_argument_types(memo)

/home/karthikeyan/stuff/python/myenv/lib/python3.10/site-packages/typeguard/__init__.py:1032: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

memo = <typeguard._CallMemo object at 0x7fd9fc7d2620>

    def check_argument_types(memo: Optional[_CallMemo] = None) -> bool:
        """
        Check that the argument values match the annotated types.
    
        Unless both ``args`` and ``kwargs`` are provided, the information will be retrieved from
        the previous stack frame (ie. from the function that called this).
    
        :return: ``True``
        :raises TypeError: if there is an argument type mismatch
    
        """
        if memo is None:
            # faster than inspect.currentframe(), but not officially
            # supported in all python implementations
            frame = sys._getframe(1)
    
            try:
                func = find_function(frame)
            except LookupError:
                return True  # This can happen with the Pydev/PyCharm debugger extension installed
    
            memo = _CallMemo(func, frame.f_locals)
    
        for argname, expected_type in memo.type_hints.items():
            if argname != 'return' and argname in memo.arguments:
                value = memo.arguments[argname]
                description = 'argument "{}"'.format(argname)
                try:
                    check_type(description, value, expected_type, memo)
                except TypeError as exc:  # suppress unnecessarily long tracebacks
>                   raise TypeError(*exc.args) from None
E                   TypeError: type of argument "arg" must be int; got float instead

/home/karthikeyan/stuff/python/myenv/lib/python3.10/site-packages/typeguard/__init__.py:875: TypeError
================================== short test summary info ==================================
FAILED tests/test_foo.py::test_foo[456.789] - TypeError: type of argument "arg" must be in...
================================ 1 failed, 1 passed in 0.14s ================================