Conditional Statements (if / elif / else)

Conditional statements let your program decide what to do based on data.

The basic if

age = 18

if age >= 18:
    print("You can vote")

If the condition is True, the indented block runs. If False, it's skipped.

if + else

temperature = 32

if temperature > 30:
    print("It's hot — drink water")
else:
    print("Weather is fine")

if + elif + else

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

score = 78

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} → Grade {grade}")

Only the first matching branch runs. The rest are skipped.

Nested if

You can put if inside if:

age = 22
has_license = True

if age >= 18:
    if has_license:
        print("Can drive")
    else:
        print("Adult but no license")
else:
    print("Too young to drive")

Combine conditions with and / or

Often cleaner than nesting:

age = 22
has_license = True

if age >= 18 and has_license:
    print("Can drive")
elif age >= 18:
    print("Adult but no license")
else:
    print("Too young to drive")

One-liner: ternary expression

age = 17
status = "Adult" if age >= 18 else "Minor"
print(status)

Same as:

if age >= 18:
    status = "Adult"
else:
    status = "Minor"

Good for short choices. Avoid for complex logic — gets unreadable.

Truthy / Falsy values

In a condition, Python treats these as False: - False - None - 0, 0.0 - Empty "", [], (), {}, set()

Everything else is True.

items = []

if items:
    print("Has items")
else:
    print("List is empty")    # this runs

# Equivalent to:
if len(items) > 0:
    print("Has items")

This lets you write cleaner code:

name = ""

# Pythonic
if not name:
    print("Name is missing")

# Not as Pythonic
if name == "" or name is None:
    print("Name is missing")

Common pitfall — == vs =

= is assignment. == is comparison.

x = 5
if x == 5:     # comparison — correct
    print("x is five")

# x = 5  inside an `if` is a SyntaxError in Python (good — protects you)

Example — leap year checker

A year is a leap year if: - divisible by 4 AND - NOT divisible by 100, OR - divisible by 400.

year = 2024

if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
    print(f"{year} is a leap year")
else:
    print(f"{year} is NOT a leap year")

Try changing year to 1900, 2000, 2023 and re-run.

Practice

What does this print?

Expected: B

score = 85
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("F")

Print 'adult' when age is 18 or more

Expected: adult

age = 18
if age > 18:                # bug: should be >= for "18 or more"
    print("adult")
else:
    print("minor")

Quiz — Quick check

What you remember

Q1. Which of these values is truthy in Python?

• 0
• []
• ""
• "False"

Why: "False" is a non-empty string, so it's truthy. The string contains the letters F-a-l-s-e but Python only sees a non-empty sequence of characters.

Q2. What does this print?

age = 17
print("Adult" if age >= 18 else "Minor")

• Adult
• Minor
• True
• SyntaxError

Why: This is a ternary expression. age >= 18 is False (17 isn't ≥ 18), so the else branch is taken.

Q3. In an if / elif / elif / else chain, how many branches run?

• All branches whose condition is True
• Only the first branch whose condition is True (or else if none match)
• Only the last branch whose condition is True
• Always just one — else

Why: Python evaluates top-down and runs the first matching branch only. Subsequent branches are skipped, even if their conditions would also be True.

Common doubts

Why use elif instead of stacking if after if?

elif is mutually exclusive — only one branch runs. Stacked ifs evaluate independently, so multiple branches can run. If you're picking one option from a set (like a grade), use elif. If you're applying multiple independent rules, use separate ifs.

Can I do if 1 < x < 10?

Yes! Python supports chained comparisons. 1 < x < 10 is the same as 1 < x and x < 10. It's clearer and slightly faster (x is only evaluated once).

How do I do a switch/case statement?

Python 3.10+ has match/case:

match status:
    case "ok": ...
    case "error": ...
    case _: ...           # default

Before 3.10, use a dictionary lookup or an if/elif chain.

What's next

→ Loops — for, while, break, continue