Conditions: if, elif, else

You learn to make your programs take decisions with if, elif, else, and logical operators.

8 min · 3 questions

A program often has to choose between several paths. The if keyword runs a block of instructions only if a condition is true. A condition is an expression that evaluates to True or False. The if line ends with a colon :, and the block that follows is indented by 4 spaces: this indentation tells Python which lines belong to the block.

PYTHON

age = 20
if age >= 18:
    print("You are an adult.")

The message is displayed only if the condition is true.

Conditions are written with comparison operators: == (equal to), != (not equal to), < (less than), > (greater than), <= and >=. To chain several cases, add elif ("else if") which tests another condition, then else ("otherwise") which runs when everything else is false.

PYTHON

temperature = 15
if temperature > 25:
    print("It's hot.")
elif temperature > 10:
    print("It's mild.")
else:
    print("It's cold.")

With 15, the first condition is false and the second is true: displays "It's mild."

To combine several conditions, use the logical operators: and (true if both conditions are true), or (true if at least one is true), and not (inverts the result: not True is False).

PYTHON

age = 30
has_ticket = True
if age >= 18 and has_ticket:
    print("Entry allowed.")
if not has_ticket:
    print("Ticket missing.")

The first message is displayed, not the second.

Don't confuse = and ==. A single = stores a value in a variable; a double == compares two values. Writing if age = 18: causes a syntax error: it's if age == 18: that you need to write.

Knowledge check

Make sure you remember the key points of this lesson.

Which operator tests equality between two values?
- =
- ==
- !=
- <=
When is the else block run?
- Every time the program runs
- When the if condition (and those of the elifs) are false
- When the if condition is true
What does the expression True and False evaluate to?
- True
- False
- An error