ruff/crates/red_knot_python_semantic/resources/mdtest/narrow/assert.md

Narrowing with assert statements

assert a value is None or is not None

def _(x: str | None, y: str | None):
    assert x is not None
    reveal_type(x)  # revealed: str
    assert y is None
    reveal_type(y)  # revealed: None

assert a value is truthy or falsy

def _(x: bool, y: bool):
    assert x
    reveal_type(x)  # revealed: Literal[True]
    assert not y
    reveal_type(y)  # revealed: Literal[False]

assert with is and == for literals

from typing import Literal

def _(x: Literal[1, 2, 3], y: Literal[1, 2, 3]):
    assert x is 2
    reveal_type(x)  # revealed: Literal[2]
    assert y == 2
    reveal_type(y)  # revealed: Literal[1, 2, 3]

assert with isinstance

def _(x: int | str):
    assert isinstance(x, int)
    reveal_type(x)  # revealed: int

assert a value in a tuple

from typing import Literal

def _(x: Literal[1, 2, 3], y: Literal[1, 2, 3]):
    assert x in (1, 2)
    reveal_type(x)  # revealed: Literal[1, 2]
    assert y not in (1, 2)
    reveal_type(y)  # revealed: Literal[3]