How to deal with *args/**kwargs in decorator?

[waxlamp]

(mypy is a blessing, it has made my programming life far more enjoyable; thank you!)

I am trying to write a generic decorator for a class of functions that take an initial, named argument, followed by arbitrary other arguments. Here's a dummy implementation, showing an "identity" decorator:

F = Callable[[Arg(str, "workspace"), VarArg(), KwArg()], Any]

def deco(f: F) -> F:
    def wrapper(workspace: str, *args: Any, **kwargs: Any) -> Any:
        return f(workspace, *args, **kwargs)

    return wrapper

When I try to use this decorator as follows:

@deco 
def do_something(workspace: str) -> Any:
   pass

I get the following error from mypy:

error: Argument 1 to "do_something" has incompatible type "Callable[[str], Any]"; expected "Callable[[str, VarArg(Any), KwArg(Any)], Any]"

Shouldn't do_something be compatible with the VarArg-laden Callable type? If not, what can I do to make this pass typechecking?

I also tried using a Protocol for this:

class WorkspaceFn(Protocol):
    def __call__(self, workspace: str, *args: Any, **kwargs: Any) -> Any: ...

Replacing the F = ... line from above with F = WorkspaceFn yields this similar error message:

error: Argument 1 to "do_something" has incompatible type "Callable[[str], Any]"; expected "WorkspaceFn"

[ilevkivskyi]

Technically mypy is correct, because (str) -> Any is not a subtype of (str, *Any, **Any) -> Any. But it is pretty typical to use *args, **kwargs as a synonym for "don't care". So essentially this is no different than #5876

For now you can use:

F = TypeVar('F', bound=Callable[..., Any])
def deco(f: F) -> F:
    ...