Python Conditionals: if, elif, else and match

Until now your programs ran every line, always in the same order. Conditionals let them make decisions: do one thing if a condition is true and something else if it isn't. They're the first of Python's control structures, and they're how you turn business rules ("free shipping over $75," "5% commission above $100,000") into code.

if: run something only when a condition is true

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

if age >= 18:
    print("You're 18 or older")
    print("You can register to vote")

print("End of program")

Three syntax details you can't skip:

1. The condition ends with a colon :.
2. The instructions that depend on the if are indented 4 spaces. That's the block.
3. The first line without indentation marks the end of the block. "End of program" always prints.

if / else: two paths

gift_card_balance = 120
purchase = 150

if gift_card_balance >= purchase:
    gift_card_balance -= purchase
    print(f"Purchase approved. Balance: ${gift_card_balance}")
else:
    short = purchase - gift_card_balance
    print(f"Insufficient balance. You're ${short} short")

Exactly one of the two blocks runs. With these numbers, the output is Insufficient balance. You're $30 short.

if / elif / else: several paths

elif is short for "else if." Python checks the conditions from top to bottom and runs only the first block whose condition is true. Letter grades are the classic example:

score = float(input("Score (0-100): "))

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

print(grade)

Compound conditions: and, or, not

age = 25
has_license = True
is_member = False
order_total = 80

if age >= 21 and has_license:
    print("Can rent the car")

if is_member or order_total > 50:
    print("Free shipping")

if not has_license:
    print("You need a driver's license first")

For ranges, Python lets you chain comparisons in a very readable way: if 18 <= age < 65:.

Truthy and falsy values

Inside a condition, Python treats these values as false: False, 0, 0.0, the empty string "", empty collections ([], {}) and None. Everything else counts as true. That lets you write clean checks like this one, which fixes the IndexError from lesson 3:

name = input("Name: ").strip()
if not name:
    print("You didn't enter a name")

Nested conditionals

You can put an if inside another one by adding indentation:

if user_registered:
    if password_correct:
        print("Welcome back")
    else:
        print("Wrong password")
else:
    print("User not found")

Use nesting sparingly: more than two or three levels make code hard to follow. Often you can combine conditions with and or reorder them instead.

The ternary expression: if on one line

status = "Pass" if score >= 60 else "Fail"
shipping = 0 if order_total >= 50 else 7.99

Use it only for simple two-option decisions. If you need an elif, go back to the full form.

match: compare against many values (Python 3.10+)

When you compare one variable against several specific values, match reads more clearly than a chain of elif statements. Here's the menu of a simple banking app:

option = input("Choose 1, 2 or 3: ")

match option:
    case "1":
        print("Check balance")
    case "2":
        print("Transfer money")
    case "3":
        print("Exit")
    case _:
        print("Invalid option")

The _ case means "any other value." Typing 2 prints Transfer money; typing 7 prints Invalid option.

Full example: an online store's shipping calculator

total = float(input("Order total: $"))
zone = input("Zone (local / in-state / out-of-state): ").strip().lower()
is_member = input("Rewards member? (y/n): ").strip().lower() == "y"

if is_member or total >= 75:
    shipping = 0
elif zone == "local":
    shipping = 5.99
elif zone == "in-state":
    shipping = 8.99
elif zone == "out-of-state":
    shipping = 12.99
else:
    shipping = None

if shipping is None:
    print("Unknown zone")
else:
    print(f"Shipping: ${shipping:.2f}")
    print(f"Total due: ${total + shipping:.2f}")

Order total: $62.50
Zone (local / in-state / out-of-state): in-state
Rewards member? (y/n): n
Shipping: $8.99
Total due: $71.49

Look at the is_member line: the comparison == "y" already returns True or False, so we store it directly as a boolean. To compare with None, use is, not ==. And because the zone goes through .strip().lower(), a customer who types "In-State " still gets the right rate.

Real case: tiered sales commissions at a Chicago distributor

Lakeshore Supply Co., a distributor in Chicago, pays its sales reps a commission that steps up with monthly sales, plus a $750 bonus for landing at least 3 new clients and reaching the minimum sales tier. Here's how Python decides which rate applies:

1. First questionSales ≥ $100,000?If True, the rate is 5% and Python skips all the other conditions.
2. OtherwiseSales ≥ $60,000?3% rate. Getting here already means sales were under $100,000.
3. OtherwiseSales ≥ $30,000?1.5% rate.
4. elseNothing matchedRate 0: the rep didn't reach the minimum tier.

# Monthly commission - Lakeshore Supply Co. (Chicago, Illinois)
sales = int(input("Sales this month ($): "))
new_clients = int(input("New clients: "))

if sales >= 100_000:
    rate = 0.05
elif sales >= 60_000:
    rate = 0.03
elif sales >= 30_000:
    rate = 0.015
else:
    rate = 0

commission = sales * rate
bonus = 750 if new_clients >= 3 and sales >= 30_000 else 0
total = commission + bonus

print(f"Rate applied: {rate:.1%}")
print(f"Commission: ${commission:,.2f}")
print(f"Bonus: ${bonus:,.2f}")
print(f"Total payout: ${total:,.2f}")

Here's what the terminal shows when Megan enters her numbers (what she types appears after each prompt):

Sales this month ($): 117500
New clients: 4
Rate applied: 5.0%
Commission: $5,875.00
Bonus: $750.00
Total payout: $6,625.00

Two new details. 100_000 is the same as 100000, but the underscores make big numbers readable at a glance (Python ignores them). And the bonus uses a ternary with and: both conditions are required. These are the month's results for the four reps, who sold $117,500, $71,000, $44,500 and $27,000 (green bars earned the bonus):

Valeria sold less than Derek (who landed only 1 new client) and earns less, but she gets the bonus because she brought in 3 new clients. Tyler brought in 5, yet gets no bonus: he didn't reach $30,000, and and requires both conditions.

Common mistakes with conditionals

Most if bugs are syntax errors, and Python describes them precisely. For example, if you forget to indent the line inside the if:

  File "commission.py", line 3
    print("ok")
    ^^^^^
IndentationError: expected an indented block after 'if' statement on line 2

Python points at line 3 because that's where it expected the indentation. These are the mistakes that come up again and again:

weight = float(input("Weight (lb): "))
height = float(input("Height (in): "))

if height <= 0:
    print("Height must be greater than 0")
else:
    bmi = 703 * weight / height ** 2
    if bmi < 18.5:
        category = "Underweight"
    elif bmi < 25:
        category = "Healthy weight"
    elif bmi < 30:
        category = "Overweight"
    else:
        category = "Obesity"
    print(f"BMI: {bmi:.1f} → {category}")

Summary

• if, elif and else end with :, and their block is indented.
• Only the first block whose condition is true runs, so order your conditions carefully.
• Combine conditions with and, or and not; compare with None using is.
• Empty strings, empty collections, 0 and None are falsy; match is cleaner for many specific values.
• Always check the colon, the indentation and that each side of an or is a full comparison.

Next lesson: Python for and while Loops: range, break and continue. You'll loop over collections, use range() and accumulators, and control loops with while, break, continue and list comprehensions.