#3142: Improve usability of multi() and multi_future() type annotations

[jrheard-seesaw]

This diff resolves #3142 .

Minimal repro:

from tornado import gen


async def foo() -> int:
    return 1


async def bar() -> str:
    return "hello"


awaitables = [foo(), bar()]

gen.multi(awaitables)

Before patch:

🚂  mypy repro.py
/Users/jrheard/dev/tornado/tornado/tcpclient.py:274: error: Argument 1 to "_create_stream" of "TCPClient" has incompatible type "int | None"; expected "int"  [arg-type]
/Users/jrheard/dev/tornado/tornado/curl_httpclient.py:366: error: Argument 2 to "_curl_header_callback" of "CurlAsyncHTTPClient" has incompatible type "Callable[[str], None] | None"; expected "Callable[[str], None]"  [arg-type]
repro.py:14: error: Argument 1 to "multi" has incompatible type "list[Coroutine[Any, Any, object]]"; expected "list[Awaitable[Any] | list[Awaitable[Any]] | dict[Any, Awaitable[Any]] | Future[Any] | None] | dict[Any, Awaitable[Any] | list[Awaitable[Any]] | dict[Any, Awaitable[Any]] | Future[Any] | None]"  [arg-type]
repro.py:14: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
repro.py:14: note: Consider using "Sequence" instead, which is covariant
Found 3 errors in 3 files (checked 1 source file)

After patch:

🚂  mypy repro.py
/Users/jrheard/dev/tornado/tornado/tcpclient.py:274: error: Argument 1 to "_create_stream" of "TCPClient" has incompatible type "int | None"; expected "int"  [arg-type]
/Users/jrheard/dev/tornado/tornado/curl_httpclient.py:366: error: Argument 2 to "_curl_header_callback" of "CurlAsyncHTTPClient" has incompatible type "Callable[[str], None] | None"; expected "Callable[[str], None]"  [arg-type]
Found 2 errors in 2 files (checked 1 source file)

I'm not sure what to make of those two remaining type errors - they appear to me to be pre-existing. LMK if I'm confused about that please!

CCing @bdarnell because I notice you commented on the original issue :)

Thanks for your work on this project!

[bdarnell]

I'm not sure what to make of those two remaining type errors - they appear to me to be pre-existing. LMK if I'm confused about that please!

I'd bet it's a mypy version issue. We're clean with the version of mypy in tornado's requirements.txt but we don't upgrade very frequently.

[jrheard-seesaw]

That makes sense, thanks! When I pin my repro repo to use the same mypy version as tornado, I now get this output:

🚂  mypy repro.py
Success: no issues found in 1 source file

Added a commit to fix an issue where I had mistakenly imported Sequence within the TYPE_CHECKING check - my mistake!

[bdarnell]

Thanks!