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fix: type Markable is reduced to Any by mypy #8674

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fix: type Markable is reduced to Any by mypy #8674

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16 changes: 7 additions & 9 deletions src/_pytest/mark/structures.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
import warnings
from typing import Any
from typing import Callable
from typing import cast
from typing import Collection
from typing import Iterable
from typing import Iterator
Expand Down Expand Up @@ -264,9 +265,9 @@ def combined_with(self, other: "Mark") -> "Mark":


# A generic parameter designating an object to which a Mark may
# be applied -- a test function (callable) or class.
# be applied -- a test function (callable) or class (also callable).
# Note: a lambda is not allowed, but this can't be represented.
Markable = TypeVar("Markable", bound=Union[Callable[..., object], type])
Markable = TypeVar("Markable", bound=Callable[..., Any])


@attr.s(init=False, auto_attribs=True)
Expand Down Expand Up @@ -344,25 +345,22 @@ def with_args(self, *args: object, **kwargs: object) -> "MarkDecorator":
mark = Mark(self.name, args, kwargs, _ispytest=True)
return MarkDecorator(self.mark.combined_with(mark), _ispytest=True)

# Type ignored because the overloads overlap with an incompatible
# return type. Not much we can do about that. Thankfully mypy picks
# the first match so it works out even if we break the rules.
@overload
def __call__(self, arg: Markable) -> Markable: # type: ignore[misc]
def __call__(self, arg: Markable) -> Markable:
pass

@overload
def __call__(self, *args: object, **kwargs: object) -> "MarkDecorator":
def __call__(self, *args: Any, **kwargs: Any) -> "MarkDecorator":
pass

def __call__(self, *args: object, **kwargs: object):
def __call__(self, *args: Any, **kwargs: Any) -> Union[Markable, "MarkDecorator"]:
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Markable is an unbound TypeVar here, which usually doesn't make sense. Since the typing in covered by the overloads I chose to omit the return type here.

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Is it unbound? I figured mypy would use the overload, but I can adjust this so there is less overlap. I can force non keyword arguments but without knowing at least one required keyword argument I don't know how to tell mypy that at least one has to be provided. to force the MarkDecorator path.

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Just to follow up on the last part of your comment... As you have it mypy (possibly any type checker) will read this as -> Any. I had thought as you are on a different codebase and I noticed that mypy wasn't checking the function at all (I had left off the argument signature too), I then added the argument signature and tested that I could return anything even outside the bounds of the @overloads. This might have been fixed in mypy, but it's best to be explicit about your return type. When I had added the return annotation it required casting the return in the Markable path since it was now checking the return was not Any.

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When a function has overloads defined, the typing of the implementation function is only used for the implementation -- it is ignored in favor of the overloads everywhere else.

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Yes, that's correct, but shouldn't the function return be type checked so that future editors don't accidentally make a mistake?

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I think best we can do is add a comment.

"""Call the MarkDecorator."""
if args and not kwargs:
func = args[0]
is_class = inspect.isclass(func)
if len(args) == 1 and (istestfunc(func) or is_class):
store_mark(func, self.mark)
return func
return cast(Markable, func)
return self.with_args(*args, **kwargs)


Expand Down