Python Conditional Expressions

Basic

Python can choose a value without choosing a whole block of statements. A conditional expression puts the two possible values and the condition into one expression, so the result can be used wherever an expression is expected.

The basic shape

1
2
3
4
5

age = 20
label = "adult" if age >= 18 else "minor"
print(label)

# The expression chooses one value; it does not run two branches.

The syntax is `value_if_true if condition else value_if_false`. Python evaluates the condition first. If it is true, the first value is evaluated and returned; otherwise the second value is evaluated and returned.

It produces a value

1
2
3
4
5

temperature = 17
message = "warm" if temperature >= 20 else "cool"

print(message)
print("warm" if temperature >= 20 else "cool")

This is the important difference from the `if` statement you already know: an `if` statement controls which block of statements runs, while a conditional expression computes one value.

Choose between computed values

1
2
3
4
5
6
7
8

score = 83
result = "pass" if score >= 50 else "try again"

price = 100
discounted = price * 0.9 if score >= 90 else price

print(result)
print(discounted)

Only the selected value expression is evaluated. That makes a conditional expression useful when choosing between computations, not merely between fixed labels.

Use it inside larger expressions

1
2
3
4
5
6
7
8

items = 3
unit_price = 12

subtotal = items * unit_price
shipping = 0 if subtotal >= 30 else 5
total = subtotal + shipping

print(total)

Because the result is an ordinary value, it can participate in arithmetic, function calls, assignments, and other expressions. Keep the surrounding expression easy to read; compact syntax is not automatically better syntax.

Keep the condition easy to read

1
2
3
4
5

logged_in = True
is_admin = False

access = "admin" if logged_in and is_admin else "standard"
print(access)

If the condition becomes difficult to understand, calculate a named boolean first or use a regular `if` statement. The goal is readable Python, not the shortest possible line.

Nested conditional expressions need care

1
2
3
4
5
6

score = 72
label = ("excellent" if score >= 90 else
         "pass" if score >= 50 else
         "retry")

print(label)

Conditional expression vs if statement

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

age = 21

# Value selection: conditional expression
label = "adult" if age >= 18 else "minor"

# Statement control: if statement
if age >= 18:
    print("You may enter.")
else:
    print("You may not enter.")

A good rule is simple: if the question is “which value should I use?”, a conditional expression may fit. If the question is “which work should the program perform?”, an `if` statement is usually clearer.