about Iterable[T] and str

from typing import *

T = TypeVar("T")

def f(it: Iterable[T]) -> list[T]:
    if isinstance(it, str):
        ret = it.split(",")  # <nothing> has no attribute "split"
    else:
        ret = list(it)
    return ret

It looks ok, but I get a problem in mypy: <nothing> has no attribute "split".
Is it the only way to solve this problem that I use # type ignore?

Your Environment

• Mypy version used: 0.991
• Python version used: 3.9
• os: win11

No source file or test is named. Start by reproducing the snippet with mypy 0.991 on Python 3.9, then inspect how type narrowing handles isinstance(it, str) for Iterable[T]. Done means establishing the supported behavior or workaround and, if this is confirmed as a bug, adding a regression test.