Can't type a decorator factory that preserves signature

[JelleZijlstra]

There doesn't appear to be a way to type a decorator factory that preserves its arguments (similar to @functools.wraps(). This is related to #1927 and the doc example in http://mypy.readthedocs.io/en/latest/generics.html#declaring-decorators but covers a slightly more complicated case.

Example code:

$ cat decorator_factory.py 
from typing import Any, Awaitable, Callable, cast, Dict, Tuple, TypeVar

FuncType = Callable[..., Any]
F = TypeVar('F', bound=FuncType)


def memoize() -> Callable[[F], F]:
    def decorator(fn: F) -> F:
        def wrapper(*args: Any, **kwargs: Any) -> Any:
            return fn(args, **kwargs)
        return cast(F, wrapper)

    reveal_type(decorator)
    return cast(Callable[[F], F], decorator)

reveal_type(memoize)
reveal_type(memoize())


@memoize()
def f() -> None:
    pass
$ python3 -m mypy decorator_factory.py 
decorator_factory.py: note: In function "memoize":
decorator_factory.py:13: error: Revealed type is 'def (fn: F`-1) -> F`-1'
decorator_factory.py: note: At top level:
decorator_factory.py:16: error: Revealed type is 'def [F <: def (*Any, **Any) -> Any] () -> def (F`-1) -> F`-1'
decorator_factory.py:17: error: Revealed type is 'def (builtins.None) -> builtins.None'
decorator_factory.py:20: error: Argument 1 has incompatible type Callable[[], None]; expected None

I'm guessing mypy doesn't see the F typevars on the outer and inner functions as the same type. Strangely, it looks like the typevar is instantiated as None, even though None is outside its bounds.

I haven't been able to think of a workaround that preserves the signature correctly; any suggestions would be appreciated.

[roganov]

dupe of #1551

I haven't been able to think of a workaround that preserves the signature correctly; any suggestions would be appreciated.

There is a workaround: #1551 (comment)

[gvanrossum]

Yeah, this problem is getting annoying (though it is remarkably hard to fix). But we only need one issue to track it.