Treat Assigning Coroutine to `_` the Same as Unused Coroutine

[michaeligreenberg]

Feature

Treat assigning a function's coroutine return value to the _ variable the same as an unused coroutine.

Pitch

Mypy attempts to catch the bug of forgetting to await a coroutine by detecting unused coroutines. For example, with mypy 1.19.1:

async def foo() -> int:
    return 42

def bar() -> None:
    foo()  # error: Value of type "Coroutine[Any, Any, int]" must be used  [unused-coroutine]

It's the style convention of some organizations to assign the unused return value of a function to the _ variable. This signals to readers that the return value was intentionally ignored and not a possible bug. However, this then silences the unused-coroutine issue.

async def foo() -> int:
    return 42

def bar() -> None:
    _ = foo()  # no mypy error

I'm not sure if there is precedent of mypy treating _ differently, but I believe that practically this would aim to catch the same category of bugs as unused-coroutine.

[A5rocks]

Generally I think we should probably overhaul unused coroutine. In an ideal world, we would ensure that all non-awaited f() do not return a coroutine... but asyncio uses that style. I think mypy should flag any assignment to something that isn't Coroutine (or CoroutineType... or maybe Awaitable?) as requiring an await. What do you think?

[michaeligreenberg]

I think mypy should flag any assignment to something that isn't Coroutine (or CoroutineType... or maybe Awaitable?) as requiring an await. What do you think?

Do you mean any assignment that isn't explicitly Coroutine, etc.? So like:

_ = foo()  # would error
bar = foo() # would also error
buzz: Coroutine = foo()  # would not error

[A5rocks]

Hm, I haven't really thought about it that much... my naive rules would allow bar = foo() (but not _ = foo() because we have special casing for that already s.t. _ is Any), which is not good...

[michaeligreenberg]

I'm fine with not letting perfect being enemy of the good and treating _ as a special case:

_ = foo()  # would error
bar = foo() # would not error

If one went onto use bar unlike a Coroutine, mypy would likely catch it. And if one never used bar, many static analyzers would warn about an unused variable.