How to annotate the type of a decorator of an async function in Python 3.7?

Question

I'd like to write a decorator for an async function. How can I annotate the type of the decorator definition?

Here's an example of what I'd like to do:

from typing import TypeVar # will Awaitable help?
AsyncFn = TypeVar('AsyncFn') # how to narrow this type definition down to async functions?

def my_decorator(to_decorate: AsyncFn) -> AsyncFn:
    async def decorated(*args, **kwargs): # mypy keeps saying "Function is missing a type"
       return await to_decorate(*args, **kwargs)

@my_decorator
async def foo(bar: int) -> str:
    return f"async: {bar}"

@my_decorator
async def quux(spam: str, **eggs: str) -> None:
    return

foo(1) # should check
foo("baz") # mypy should yell
quux("spam") # should check
quux(1) # mypy should complain
quux(spam="lovely", eggs="plentiful") # ok
quux(spam=1, eggs=0) # mypy should throw a fit

Answer 1

An async function is a function that returns an Awaitable.

With ParamSpec introduced in Python 3.10 the complete type annotations for a decorator for an async function is:

from typing import Awaitable, Callable, ParamSpec, TypeVar

T = TypeVar("T")
P = ParamSpec("P")


def decorator(fn: Callable[P, Awaitable[T]]) -> Callable[P, Awaitable[T]]:
    async def decorated(*args: P.args, **kwargs: P.kwargs) -> T:
        return await fn(*args, **kwargs)

    return decorated

In Python 3.7 ParamSpec can be imported from typing_extensions.