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MyPy does not understand types exclusion using operator "is" #15788

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Real example:

from typing import Any, Collection, Generator, Iterable, TypeVar, cast
from itertools import chain, islice

T = TypeVar('T')  


def _split_on_batch_gen(items: Iterable[T]) -> Generator[Iterable[T], Any, None]:
    items = iter(items)
    stopper = object()
    while True:
        first_item = next(items, stopper)
        if first_item is stopper:
            break
        batch: chain[T] = chain((first_item,), islice(items, 0, 99))
        yield batch

or, reduced:

from typing import Any, Collection, Generator, Iterable, TypeVar, cast
from itertools import chain, islice

items = iter(range(1000))
stopper = object()
first_item = next(items, stopper)
if first_item is not stopper:
    batch: Iterable[int] = chain((first_item,), islice(items, 0, 99))

check here: https://mypy-play.net/?mypy=latest&python=3.11

gives:

main.py:8: error: Argument 1 to "chain" has incompatible type "tuple[object]"; expected "Iterable[int]"  [arg-type]

Which is obviously incorrect, since we checked that first_item is not an object(). I can not check for isinstance(fist_item, object) because every object is object :). Seems, MyPy understands isinstance() assertions, but does not understand is.

Seems, type of next(xxx, yyy) should be xxx[0] | Literal[yyy] instead of xxx[0] | yyy (pseudocode, I don't know how to express correctly)

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