What Is the Ternary Operator in Python?

Python's conditional expression, which is often called the ternary operator, is a single expression that picks one of two values. The order of the parts is the reverse of the C ternary, so it reads as value_if_true if condition else value_if_false.

age = 20
status = 'adult' if age >= 18 else 'minor'
print(status)                  # adult

# The condition does not have to be a bool, because truthiness decides.
name = ''
label = 'anonymous' if not name else name
print(label)                  # anonymous

# Any expression is allowed on either side.
cart = ['book']
message = 'you have items' if cart else 'your cart is empty'
print(message)                # you have items

# It is an expression, so you can use it inline.
print('paid' if status == 'adult' else 'unpaid')

The important property is that it is an expression and not a statement, which means that it produces a value and it can appear anywhere a value can appear. You can use it inside a call, on the right-hand side of an assignment, in a return statement, or nested inside another conditional expression.

There are two habits to warn against. The first one is that the order is easy to get backwards. Writing 'adult' if status else 'minor' tests the wrong thing, because status is a non-empty string and is therefore always truthy. The second one is the classic trap of writing value = a or b, which is not the same as a if a else b. The or form returns a whenever a is truthy, even when a is not the value you meant, whereas the conditional expression tests a directly and returns b only when a is falsy.

There is some style guidance worth repeating. Keep a conditional expression on one line and keep it short. If it needs brackets and a line break, then an if and else block is clearer.

The conditional expression is the two-value version of an if and else, and it is written as value_if_true if condition else value_if_false. It was added in Python 2.5, and it is an expression, which is the only real difference from a statement.

That single word, expression, has practical consequences. A conditional expression has a value, and that value can be assigned, returned, passed as an argument, or nested inside another expression. An if and else block is a statement, so it cannot be assigned or returned. It only works by binding names as a side effect. That is why you write return value if condition else other inside a function, and why you write the four-line if and else block when the branches contain several statements each.

The second difference is about scope and readability. An if and else block can hold any number of statements, including loops and nested branches, and it can bind names that survive after the block has finished. A conditional expression works out exactly one of two expressions, so anything with a side effect inside it happens at most once, and only for the branch that was chosen.

A third difference is that there is no elif equivalent, although you can nest conditional expressions inside the else branch. That nesting becomes hard to read very quickly, and that is the practical limit of the construct.

The rule of thumb to offer in an interview is that you use the conditional expression for a short and obvious choice between two values, and you use an if and else block for anything that needs more than a line or two of explanation. Mixing them up is one of the most common complaints in Python code reviews.