Python typings and futures

Question

I'm very glad to use typing module in Python 3. Also, I'm very glad to use asyncio instead of twisted, tornado and alternatives.

My question is how to define result of a coroutine properly?

Should we tell it's just a coroutine? Example 1:

async def request() -> asyncio.Future:
    pass

Or should we define type of result of coroutine as type of returning value? Example 2:

async def request() -> int:
    pass

If yes, then how to be with plain functions, which return futures? Example 3:

def request() -> asyncio.Future:
    f = asyncio.Future()
    # Do something with the future
    return f

Is it a right way? How then we can tell what is expected to be a result of the future?

Mendelevium

Use the generic versions, e.g. docs.python.org/3/library/typing.html#typing.Awaitable – Kreegar

Thanks, that's perfect – Mendelevium

This question is similar to: The right way to type hint a Coroutine function?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. – Dichotomize

Hello @InSync, thank you for the suggestion. Considering this question was asked 5 years before the one you've mentioned, I'd suggest you asking the author of that later question. – Mendelevium

That question has better answers, and being older does not prevent a question from being a dupe. See Should I flag a question as duplicate if it has received better answers?. – Dichotomize

Answer 1

As @jonrsharpe said, typing.Awaitable perfectly suits the task.

Mendelevium

Answer 2

In general, you should regular return value (such as int, float, bool, None and etc), but if you use it as a callable it should look like this:

async def bar(x: int) -> str:
    return str(x)

cbar: Callable[[int], Awaitable[str]] = bar

For more information: here.

You can look at this issue also for mypy support.

Aspectual

As for a type of the function, yes, that's true. But question was only about function's return type. Anyway, thank you – Mendelevium