Making decisions with if

Python checks if and each elif from the top and runs only the first branch whose condition is True; the rest are skipped.

A ticket price that depends on age

Here is a problem to try before any new syntax. A small science museum (made up for this example) charges nothing for children under 7, 3,000 won for ages 7 to 18, and 5,000 won for everyone else. Given an age, the program should print the price. Write the rule in plain words first, as a list of questions in the order you would ask them at the ticket desk.

You probably wrote something like this: is the visitor younger than 7? If yes, free, and stop. If not, are they 18 or younger? If yes, 3,000 won, and stop. Otherwise, 5,000 won. That is already the program. Python spells "if", "otherwise if" and "otherwise" as if, elif and else.

age = 12
if age < 7:
    price = 0
elif age <= 18:
    price = 3000
else:
    price = 5000
print(price)

3000

Three details of the syntax matter. Each condition line ends with a colon. The lines that belong to a branch are indented, and the Python tutorial recommends four spaces. When the indentation ends, the branch ends, which is why print(price) runs whatever the age. And only one branch runs: for age 12, the first condition is False, the second is True, so price becomes 3000 and the else is never looked at.

Comparisons give True or False

Note the double equals. One = assigns a value, two == compare. Strings can be compared too: "kimchi" == "Kimchi" is False, because capital letters count as different characters.

The order of the elif lines changes the answer

Because Python stops at the first True condition, the order of the branches matters. Grading a score is the classic case. Check for 90 and above first, then 80, then 70. Put the 70 check first and a score of 95 is caught by it and gets a C, since 95 >= 70 is also True.

score = 95
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "D"
print(grade)

A

and, or, not join conditions

and is True only when both sides are True. or is True when at least one side is. not flips True and False. A condition such as "between 13 and 18" can be written age >= 13 and age <= 18, or in Python's shorter form 13 <= age <= 18. A condition can also sit inside another branch, which is called nesting. The Korean middle-school informatics curriculum names both, logical operations and nested control structures, in one of its programming standards.

age = 15
has_ticket = True
if 13 <= age <= 18 and has_ticket:
    print("Teen entry")
if not has_ticket or age < 7:
    print("See the desk")
else:
    print("Go in")

Teen entry
Go in