Improve type signature for _RuleList

[mhils]

This PR improves the type signature for _RuleList to be more forgiving. Minimal repro:

from tornado.web import Application, RequestHandler

class SampleHandler(RequestHandler):
    def get(self):
        self.write("Hello world")

handlers = [
    (r"/", SampleHandler),
]

application = Application(handlers)

Before Patch:

$ mypy repro.py
repro.py:11: error: Argument 1 to "Application" has incompatible type "list[tuple[str, type[SampleHandler]]]"; expected "list[Rule | list[Any] | tuple[str | Matcher, Any] | tuple[str | Matcher, Any, dict[str, Any]] | tuple[str | Matcher, Any, dict[str, Any], str]] | None"  [arg-type]
repro.py:11: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
repro.py:11: note: Consider using "Sequence" instead, which is covariant
Found 1 error in 1 file (checked 1 source file)

After Patch:

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

Thank you for maintaining tornado, we (@mitmproxy) are super happy users for over a decade! 😃✨🍰

[bdarnell]

Thanks, but according to #3119 (comment) changing this List to Sequence is incorrect. I'm not sure if that's true, though - this change appears to pass our mypy tests. (the PR was submitted at a time that our CI wasn't working so we don't can't see the test results on the web but I just checked out the branch and verified it locally). What do you think about that?

[mhils]

Thanks, but according to #3119 (comment) changing this List to Sequence is incorrect.

I think that comment is a bit misguided. _RuleList is only used as an annotation when accepting arguments and it's not publicly exposed. _RuleList is never used to annotate attributes or return types. It's typically good to accept wide abstract types and return concrete ones.

It would be different if self.routes would be annotated with _RuleList, but that's already (correctly) annotated as List[Rule]. If anything, we could consider widening from Sequence to Iterable.

TL;DR: I think this is fine as-is, if anything we can make it an Iterable. :)

[bdarnell]

That makes sense to me. Thanks!