Argument 1 to "__anext__" of "AsyncIterator" has incompatible type "AsyncIterator[_T_co]"; expected "AsyncIterator[_T_co]"

[aaugustin]

To Reproduce

While making changes in websockets to account for changes in mypy 0.941 -> 0.950, I encountered this error:

Argument 1 to "__anext__" of "AsyncIterator" has incompatible type "AsyncIterator[_T_co]"; expected "AsyncIterator[_T_co]"

Regardless of what actually happens, "incompatible A, expected A" is not a helpful error message. This is why I am reporting this bug. See https://github.com/aaugustin/websockets/runs/6272618187?check_suite_focus=true for the test run where this happened.

I'm sorry, I'm not sure I will find the motivation to reduce this to a minimal test case. I'm probably going to pile cast() until mypy passes. If you're interested, you can reproduce by checking out this commit python-websockets/websockets@dedc0dd and running mypy 0.950 on it. If not, just close this.

Expected Behavior

Either no error — apparently mypy gets what it expects? — or an error message that makes sense.

Actual Behavior

A nonsensical error message.

Your Environment

• Mypy version used: 0.950 installed with pipx (so, up to date and isolated - I reinstalled it and that didn't help)
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): -
• Python version used: 3.10.2 from homebrew
• Operating system and version: macOS 12.3.1

[JelleZijlstra]

Repro case:

from typing import AsyncIterable

async def func(message: AsyncIterable[bytes | str] | bytes | str) -> None:
        if isinstance(message, AsyncIterable):
            # aiter_message = aiter(message) without aiter
            # https://github.com/python/mypy/issues/5738
            aiter_message = type(message).__aiter__(message)  # type: ignore
            try:
                # message_chunk = anext(aiter_message) without anext
                # https://github.com/python/mypy/issues/5738
                message_chunk = await type(aiter_message).__anext__(  # type: ignore
                    aiter_message
                )
            except StopAsyncIteration:
                return

https://mypy-play.net/?mypy=latest&python=3.10&gist=3c85aae085133db6d751711196fd4b76

[aaugustin]

Oh, this is actually a change in the behavior of # type: ignore comments.

This fixes the problem:

diff --git a/src/websockets/legacy/protocol.py b/src/websockets/legacy/protocol.py
index d1d52bf..ada4140 100644
--- a/src/websockets/legacy/protocol.py
+++ b/src/websockets/legacy/protocol.py
@@ -684,8 +684,8 @@ class WebSocketCommonProtocol(asyncio.Protocol):
             try:
                 # message_chunk = anext(aiter_message) without anext
                 # https://github.com/python/mypy/issues/5738
-                message_chunk = await type(aiter_message).__anext__(  # type: ignore
-                    aiter_message
+                message_chunk = await type(aiter_message).__anext__(
+                    aiter_message  # type: ignore
                 )
             except StopAsyncIteration:
                 return

I assume that was an intentional change in how # type: ignore comments work when arguments are split over multiple lines and the error affects only one argument.

[JelleZijlstra]

Interesting, I don't remember any recent changes that would affect where the error is emitted.

However, you are quite right that at best, the error message is confusing, and probably the whole thing is buggy. Your comment refers to #5738, which might be right.

[aaugustin]

I think it's fair to close as a duplicate of #5738 -- if there's something interesting in how errors are reported, someone else will find it again :-)