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If expressions

crates/ty_python_semantic/resources/mdtest/conditional/if_expression.md

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If expressions

Simple if-expression

py
def _(flag: bool):
    x = 1 if flag else 2
    reveal_type(x)  # revealed: Literal[1, 2]

If-expression with walrus operator

py
def _(flag: bool):
    y = 0
    z = 0
    x = (y := 1) if flag else (z := 2)
    reveal_type(x)  # revealed: Literal[1, 2]
    reveal_type(y)  # revealed: Literal[0, 1]
    reveal_type(z)  # revealed: Literal[0, 2]

Nested if-expression

py
def _(flag: bool, flag2: bool):
    x = 1 if flag else 2 if flag2 else 3
    reveal_type(x)  # revealed: Literal[1, 2, 3]

None

py
def _(flag: bool):
    x = 1 if flag else None
    reveal_type(x)  # revealed: Literal[1] | None

Statically known compound conditions

Short-circuit conditions can select a single branch even when an operand has mutable truthiness. Saving the condition's value and testing it again does not provide the same guarantee.

py
def _(value: object):
    reveal_type(1 if value and False else 2)  # revealed: Literal[2]
    reveal_type(1 if value or True else 2)  # revealed: Literal[1]

    saved = value and False
    reveal_type(1 if saved else 2)  # revealed: Literal[1, 2]

A comparison chain can select a single branch even when an individual comparison returns an arbitrary object. Saving the chain's result allows that object's truthiness to be tested again.

py
class Comparable:
    def __lt__(self, other: int) -> object:
        return object()

def _(value: Comparable):
    reveal_type(1 if value < 1 < 0 else 2)  # revealed: Literal[2]

    saved = value < 1 < 0
    reveal_type(1 if saved else 2)  # revealed: Literal[1, 2]

An operand narrowed to Never cannot produce a result. Nested conditions preserve the remaining short-circuit outcome when selecting a branch.

py
def _(other: object, value: bool):
    reveal_type(1 if other and (isinstance(value, str) and value) else 2)  # revealed: Literal[2]
    reveal_type(1 if other or (not isinstance(value, str) or value) else 2)  # revealed: Literal[1]

Conditions with operands equivalent to Never

A call whose return type is an alias of Never cannot produce a result. The preceding short-circuit outcome alone selects the conditional expression's branch.

toml
[environment]
python-version = "3.12"
py
from typing import Never

type Bottom = Never

def stop() -> Bottom:
    raise RuntimeError

def _(flag: bool):
    reveal_type(1 if flag and stop() else 2)  # revealed: Literal[2]
    reveal_type(1 if flag or stop() else 2)  # revealed: Literal[1]

A type variable bounded by Never also cannot produce a result.

py
def _[T: Never](flag: bool, value: T):
    reveal_type(1 if flag and value else 2)  # revealed: Literal[2]

Condition with object that implements __bool__ incorrectly

py
class NotBoolable:
    __bool__: int = 3

# error: [unsupported-bool-conversion] "Boolean conversion is not supported for type `NotBoolable`"
3 if NotBoolable() else 4