Conditionals

Every program eventually needs to make a decision — charge sales tax or not, show an error or not, sort someone into one category instead of another. if is how Python does that.

if, elif, and else

Python checks each condition top to bottom and runs the first block whose condition is true, skipping the rest. else catches anything that didn't match:

score = 82

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(grade)

B

82 fails the first check (it's not 90 or above), so Python moves to the next line. It passes score >= 80, sets grade to "B", and — this is the important part — never even looks at the remaining elif and else lines. Once one branch runs, the rest are skipped.

Combining conditions with and / or

and requires both sides to be true; or only needs one:

age = 20
has_ticket = True

if age >= 18 and has_ticket:
    print("Welcome in")
else:
    print("Not allowed")

Welcome in

Conditions can depend on more than one variable

temperature = 15
is_raining = True

if temperature < 10:
    print("Wear a coat")
elif is_raining:
    print("Bring an umbrella")
else:
    print("Enjoy the weather")

Bring an umbrella

15 isn't below 10, so the first branch is skipped, and Python falls through to check is_raining, which is True.

A shorter form for simple cases

When a condition only decides between two values, you can write it on one line instead of spreading it across four:

age = 16
status = "adult" if age >= 18 else "minor"
print(status)

minor