Clarification: return type for asyncio.gather()?

[bsolomon1124]

I've come across a source of confusion for asyncio.gather(). It may very well be user error rather than an incorrect annotation. (Motivated by this Stack Overflow post.)

The annotation return types for gather() are Future[Tuple[_T1]], Future[Tuple[_T1, _T2]], and so on.

However, the result of return await asyncio.gather(*tasks) is List. I.e.:

import asyncio
from typing import List

async def foo(x) -> int:
    await asyncio.sleep(x / 2)
    return x

async def main() -> List[int]:
    return await asyncio.gather(*(foo(i) for i in (1, 2, 3)))

if __name__ == "__main__":
    print(asyncio.run(main()))

This runs find, printing [1, 2, 3]; await asyncio.gather() produces a List[int].

However, mypy complains that:

mypytest.py:9: error: Incompatible types in assignment (expression has type "Tuple[Any, ...]", variable has type "List[int]")

Why does this ambiguity exist? How can I provide a correct type hint for async def main()?

---

I'm looking at typeshed master branch as of 0602840 fyi + mypy 0.720.

[srittau]

I assume it's done like this in the stubs, because typing's type system only allows Lists with consistent types, as opposed to Tuples. This allows you to write code like this and have it correctly type-checked:

x, y, z = await gather(foo(), bar(), baz())

Where foo, bar, and baz each return a different type.

But this should at least be documented in the stubs.

[bsolomon1124]

@srittau Yes, the iterable unpacking portion of that code makes sense but, in my own view, this is a bit like calling the sky orange 😉 gather() does return a list (of potentially heterogeneous items).

Looking back though, PR #1550 discussed this specifically, followed by c5f1e90. So if we agree in something, it's that it could be documented a bit more clearly in case, for instance, someone tries to call .append() on the result of gather().

[tsafs]

I assume it's done like this in the stubs, because typing's type system only allows Lists with consistent types, as opposed to Tuples. This allows you to write code like this and have it correctly type-checked:

x, y, z = await gather(foo(), bar(), baz())

Where foo, bar, and baz each return a different type.

But this should at least be documented in the stubs.

That solved this error for me

Cannot unpack 1 value into 2 variables [bad-unpacking]