Static type check fails for proposed everything_except() recipe.

[jenstroeger]

Hello, following the example recipe given with the from_type() strategy (I removed the excluded_types: type | tuple[type] arg to simplify the code), I noticed that said recipe fails to type check:

# test.py

import typing

import hypothesis
import hypothesis.strategies as st

def everything_except() -> st.SearchStrategy[typing.Any]:
    return (
        st.from_type(type)
        .flatmap(st.from_type)
        .filter(lambda x: not isinstance(x, int))
    )

fails with this error:

test.py:1139:50: error: Cannot use a covariant type variable as a parameter  [misc]

A few observations:

• Note the line number — it’s always that line no matter where in the code I place this function.
• Placing a # type: ignore[misc] comment doesn’t work for any line in the above test.
• Removing flatmap() passes the type checks.

Running that strategy seems to work, though:

@hypothesis.given(data=everything_except())
def test_data(data: typing.Any) -> None:
    print(type(data), data)

test_data()

produces the expected output.

This is for hypothesis v6.86.1 and mypy v1.5.1. Please let me know if you need more information, or if you’re able to reproduce.

I’m going to dig around some more later today to try and find the cause of the failing type check. For the time being, I’m going to (have to) decorate the function with @no_type_check to disable type checking.

[Zac-HD]

Huh, that line number points to

hypothesis/hypothesis-python/src/hypothesis/strategies/_internal/core.py

Line 1139 in f73dab7

def from_type(thing: Type[Ex]) -> SearchStrategy[Ex]:

and sure enough, if I change Ex -> Ex_Inv the error goes away. Patch incoming!

You might also want to report a bug against mypy for the confusing location report though, claiming it's in test.py sure seems like a bug to me.

[jenstroeger]

Thank you for the instant fix @Zac-HD, I opened an issue with mypy also.

I’ll try again when the new Hypothesis release rolls out.

[Zac-HD]

Hypothesis 6.87.0 has the fix and was released three hours ago 😁

[jenstroeger]

Indeed it worked 👍🏼