Ternary Expressions

Ternaries are a great way to reduce a series of statements, like an if/else block, to a single expression. When you first learned how to use conditional logic in Python, it probably looked like this:

result: float = 0
if number % 2 == 0:
    result = number / 2
else:
    result = (number * 3) + 1

This code sets result to a dummy value like 0 (None would also work), then overwrites it with its "real" value based on the condition. A ternary lets us do all that in one expression:

result: float = number / 2 if number % 2 == 0 else (number * 3) + 1

Note that we also avoided mutating the result variable! Ternary expressions are good for maintaining immutability.

The syntax for a ternary in Python is:

value_a if condition else value_b

This qualifies as an expression because it's a single statement that evaluates to a value – one of two values, depending on the condition.

Because it's an expression, you can use it anywhere a value is expected, including as a return statement:

def get_discount(is_member: bool) -> float:
    return 0.1 if is_member else 0.0

msg: str = (
    "Access granted"
    if (
        user.is_authenticated
        and (user.role == "admin" or (user.role == "editor" and not user.suspended))
    )
    else ("Access limited" if user.is_authenticated else "Access denied")
)