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Narrowing in boolean expressions

crates/ty_python_semantic/resources/mdtest/narrow/boolean.md

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Narrowing in boolean expressions

In or expressions, the right-hand side is evaluated only if the left-hand side is falsy. So when the right-hand side is evaluated, we know the left side has failed.

Similarly, in and expressions, the right-hand side is evaluated only if the left-hand side is truthy. So when the right-hand side is evaluated, we know the left side has succeeded.

Narrowing in or

py
class A: ...

def _(x: A | None):
    isinstance(x, A) or reveal_type(x)  # revealed: None
    x is None or reveal_type(x)  # revealed: A
    reveal_type(x)  # revealed: A | None

Narrowing in and

py
from typing import final

class A: ...

def _(x: A | None):
    isinstance(x, A) and reveal_type(x)  # revealed: A
    x is None and reveal_type(x)  # revealed: None
    reveal_type(x)  # revealed: A | None

@final
class FinalClass: ...

# We know that no subclass of `FinalClass` can exist,
# therefore no subtype of `FinalClass` can define `__bool__`
# or `__len__`, therefore `FinalClass` can safely be considered
# always-truthy, therefore this always resolves to `None`
reveal_type(FinalClass() and None)  # revealed: None

Multiple and arms

py
class A: ...

def _(x: A | None, flag1: bool, flag2: bool):
    flag1 and isinstance(x, A) and reveal_type(x)  # revealed: A
    isinstance(x, A) and flag1 and reveal_type(x)  # revealed: A
    reveal_type(x) and isinstance(x, A) and flag2  # revealed: A | None

Multiple or arms

py
class A: ...

def _(x: A | None, flag1: bool, flag2: bool, flag3: bool):
    flag1 or isinstance(x, A) or reveal_type(x)  # revealed: None
    isinstance(x, A) or flag2 or reveal_type(x)  # revealed: None
    reveal_type(x) or isinstance(x, A) or flag3  # revealed: A | None

Multiple predicates

py
from typing import Literal

class A: ...

def _(x: A | None | Literal[1]):
    x is None or isinstance(x, A) or reveal_type(x)  # revealed: Literal[1]

Mix of and and or

py
from typing import Literal

class A: ...

def _(x: A | None | Literal[1]):
    isinstance(x, A) or x is not None and reveal_type(x)  # revealed: Literal[1]