ruff/crates/ty_python_semantic/resources/mdtest/narrow/while.md

Narrowing in while loops

We only make sure that narrowing works for while loops in general, we do not exhaustively test all narrowing forms here, as they are covered in other tests.

Note how type narrowing works subtly different from if ... else, because the negated constraint is retained after the loop.

Basic while loop

def next_item() -> int | None:
    return 1

x = next_item()

while x is not None:
    reveal_type(x)  # revealed: int
    x = next_item()

reveal_type(x)  # revealed: None

while loop with else

def next_item() -> int | None:
    return 1

x = next_item()

while x is not None:
    reveal_type(x)  # revealed: int
    x = next_item()
else:
    reveal_type(x)  # revealed: None

reveal_type(x)  # revealed: None

Nested while loops

from typing import Literal

def next_item() -> Literal[1, 2, 3]:
    raise NotImplementedError

x = next_item()

while x != 1:
    reveal_type(x)  # revealed: Literal[2, 3]

    while x != 2:
        # TODO: this should be Literal[1, 3]; Literal[3] is only correct
        # in the first loop iteration
        reveal_type(x)  # revealed: Literal[3]
        x = next_item()

    x = next_item()

With break statements

def next_item() -> int | None:
    return 1

while True:
    x = next_item()
    if x is not None:
        break

reveal_type(x)  # revealed: int