How to make mypy ignore pytest.approx in a code outside of test functions

I want to use pytest.approx(...) inside immutable dataclasses (frozen=True) in my unittest, so I can use a single assert to check possibly quite complex structures.

This works fine when I use these structures inside test functions, but when I declare a global variable/constant which uses approx, I get an error in mypy.

Example code:

from pytest import approx
from typing import Final
from dataclasses import dataclass

@dataclass(frozen=True)
class A:
    x: float


EXPECTED_VALUE_1: Final = A(approx(0.5)) # <- this is the problem

def test_1():
    assert A(0.5) == EXPECTED_VALUE_1

def test_2():
    expected_value_2 = A(approx(0.5)) # <- this is fine
    assert A(0.5) == expected_value_2

The error:

$ mypy .
a.py:10: error: Argument 1 to "A" has incompatible type "ApproxBase"; expected "float"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

I think I know why - mypy by default ignores untyped functions (and my unittest functions are all untyped), but outside those functions it checks.

I've found 2 solutions:

1. add # type: ignore where this happens - which is in my opinion ugly and makes the code less readable
2. using pytest.fixture in place of a global constant. But in many cases, I have a lot of these EXPECTED_... constants (which are using approx) and converting them to fixtures makes code a lot longer and less readable, so, for simple, immutable data, I would like to keep using these global constants.

Can I somehow configure mypy to just ignore every call to approx and check everything else?

Configuration:

• Python 3.10.16

• mypy 1.16.0

• mypy configuration:

  [mypy]
  python_version = 3.10
  ignore_missing_imports = True
  

Suggested solutions/duplicate questions:

• How can mypy ignore a single line in a source file? - this is dealing with ignoring one line of code by adding a # type: ignore, which is something I wrote in "What I tried" section I know about and that it's not a solution I want. I want a solution where I would, if possible, change mypy config or somehow "patch" approx so mypy would ignore just the approx calls but otherwise check typing errors.

You can go with duck-like-typing and define statically different version of approx:

from typing import TYPE_CHECKING
if TYPE_CHECKING:
    def approx[T](expected: T, rel=None, abs=None, nan_ok=False) -> T: ...
else:
    from pytest import approx

With this approx(0.5) will viewed as a float by your type-checker, at runtime the normal ApproxBase is used.