chapter 4.1

Control Flow

Making decisions with conditionals

So far, your code runs top to bottom, every line, every time. Conditionals let your code make decisions — run this block *only if* something is true.

temperature = 35

if temperature > 30:
    print("It's hot outside!")
else:
    print("It's nice out.")

⚠warning

Indentation matters! The indented lines under if and else are the body of each branch. Python uses indentation (4 spaces) instead of braces {} like other languages.

Use elif (short for "else if") to check multiple conditions in order:

score = 85

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

print(f"Score: {score} → Grade: {grade}")

Comparison operators return True or False:

◇info

== equal to | != not equal to
> greater than | < less than
>= greater or equal | <= less or equal

Logical operators combine conditions:

age = 25
has_id = True

if age >= 21 and has_id:
    print("Access granted")

temperature = 15
if temperature < 0 or temperature > 40:
    print("Extreme weather!")
else:
    print("Weather is fine")

is_raining = False
if not is_raining:
    print("No umbrella needed")

You can nest conditionals, but keep it readable — if you're more than 2-3 levels deep, there's probably a cleaner way:

logged_in = True
is_admin = False

if logged_in:
    if is_admin:
        print("Welcome, admin")
    else:
        print("Welcome, user")
else:
    print("Please log in")

your turn

challenge

Build a grading system: given a `score` variable, print the grade (A/B/C/D/F) with a message. Use `elif` for each threshold and an f-string for the output.

loading editor...

output

$ Press RUN to execute your code

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