if / elif / else

An if statement runs a block of code only when a condition is true.

age = int(input("Age: "))

if age >= 18:
    print("You may enter.")
    print("Enjoy the show.")

print("Goodbye")

Three details matter here:

1. The condition ends with a colon.
2. The lines that belong to the if are indented — four spaces, by convention.
3. print("Goodbye") is not indented, so it runs either way.

Indentation is how Python marks a block. Other languages use braces; Python uses the layout you would have used anyway.

else and elif

else catches everything the if did not. elif (“else if”) adds another question, and you can chain as many as you like.

score = int(input("Score out of 100: "))

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Score {score} earns a {grade}")

Order matters enormously. Python checks the branches top to bottom and stops at the first true one — so a score of 95 never reaches the >= 80 test. Write the chain the wrong way round (>= 60 first) and every passing score becomes a D.

n = 15

print("With elif:")
if n % 3 == 0:
    print("  divisible by 3")
elif n % 5 == 0:
    print("  divisible by 5")

print("With separate ifs:")
if n % 3 == 0:
    print("  divisible by 3")
if n % 5 == 0:
    print("  divisible by 5")

Nesting, and how to avoid it

An if can contain another if. It can, but deeply nested conditions get hard to follow fast.

username = input("Username: ")
password = input("Password: ")

# Nested: the happy path is buried at the deepest indent.
if username:
    if password:
        if len(password) >= 6:
            print("Welcome back")
        else:
            print("Password too short")
    else:
        print("Password required")
else:
    print("Username required")

The same logic reads far better as a series of early exits, each handling one problem and getting out of the way:

username = input("Username: ")
password = input("Password: ")

if not username:
    print("Username required")
elif not password:
    print("Password required")
elif len(password) < 6:
    print("Password too short")
else:
    print("Welcome back")

Same behaviour, one level of indentation, and every rule sits on its own line.

The conditional expression

When a whole if/else exists only to choose between two values, Python has a one-line form:

count = 1

label = "item" if count == 1 else "items"
print(f"{count} {label}")

temperature = 31
print("hot" if temperature > 30 else "not hot")

Use it for short, obvious choices. If you find yourself nesting one inside another, go back to a normal if.

n = int(input("Number: "))

# Your code here

n = int(input("Number: "))

if n % 3 == 0 and n % 5 == 0:
    print("FizzBuzz")
elif n % 3 == 0:
    print("Fizz")
elif n % 5 == 0:
    print("Buzz")
else:
    print(n)

What you learned

• if condition: runs an indented block when the condition is true.
• elif adds alternatives; else catches the rest; exactly one branch runs.
• Order your conditions from most specific to least.
• Guard clauses beat deep nesting.
• a if condition else b chooses between two values inline.