Making decisions — if, elif, else

So far every program you have written does exactly the same thing every time you run it. That is fine for a calculator, but real programs need to respond — to show a different message depending on a user's score, to skip a step when a value is missing, to keep asking for input until the user types something valid.

The tool for this is the conditional statement: a branch in the road where the program checks a condition and takes one path or the other.

if, elif, else

The simplest conditional checks one condition:

temperature = 22

if temperature > 30:
    print("It is hot today.")

If the condition temperature > 30 is True, the indented block runs. If it is False, the block is skipped entirely.

Add else for a fallback:

if temperature > 30:
    print("It is hot today.")
else:
    print("Not too hot.")

Add elif (short for "else if") to check multiple conditions in sequence:

if temperature > 30:
    print("Hot")
elif temperature > 15:
    print("Comfortable")
elif temperature > 0:
    print("Cold")
else:
    print("Freezing")

Python checks each condition in order and runs only the first branch that matches. Once one branch runs, the rest are skipped.

Comparison operators

Conditions are built from comparisons that produce True or False:

Note the difference between == (comparison, asks a question) and = (assignment, sets a variable). Confusing them is one of the most common beginner mistakes.

Combining conditions with and, or, not

You can join conditions:

age = 20
has_ticket = True

if age >= 18 and has_ticket:
    print("You may enter.")

if not has_ticket:
    print("Please buy a ticket first.")

• and — both conditions must be True
• or — at least one condition must be True
• not — flips True to False and vice versa

Edit the value of temp and run again to see which branch activates.

Exercise

Where to go next

Next: loops — repeating an action many times without copying code.