Stricter typing for TaskGroup.start()

[jonathanslenders]

Looking at how we type TaskGroup.start_soon, we can do the same approach to have stricter type checking on TaskGroup.start.

We can do the following:

from typing_extensions import Protocol
from typing_extensions import TypeVarTuple, Unpack, TypeVar, Generic


T = TypeVar("T")


class TaskStatus(Generic[T]):
    def ready(self, result: T) -> None:
        ...


# ---

PosArgsT = TypeVarTuple("PosArgsT")
TaskStatusResult = TypeVar("TaskStatusResult")


class _StartFunc(Protocol[Unpack[PosArgsT], TaskStatusResult]):
    async def __call__(
        self, *args: Unpack[PosArgsT], task_status: TaskStatus[TaskStatusResult]
    ) -> None:
        ...


class TaskGroup:
    async def start(
        self,
        func: _StartFunc[Unpack[PosArgsT], TaskStatusResult],
        *args: Unpack[PosArgsT],
    ) -> TaskStatusResult:
        ...

Example usage::

async def task(a: int, b: str, task_status: TaskStatus[bool]) -> None:
    task_status.ready(True)

async def main() -> None:
    tg = TaskGroup()
    r: float = await tg.start(task, 6, "t")

On the last line, typing the result as float is still important, otherwise it will be Any. Mypy's type inference is lacking here. But if we would type it as str, then mypy would say:

error: Argument 1 to "start" of "TaskGroup" has incompatible type "Callable[[int, str, TaskStatus[bool]], Coroutine[Any, Any, None]]"; expected "_StartFunc[int, str, str]"  [arg-type]

That is significantly better than object what we currently have here:
https://github.com/agronholm/anyio/blob/master/src/anyio/abc/_tasks.py#L70

What do you think?

[jonathanslenders]

I just see previous attempts have been made to supply types for start().

See: #511
And most recent mypy issue: python/mypy#16522

When trying to finalize a PR, I was hitting exactly those issues, so I guess we have to wait for mypy...