Python If/Else

Programs become useful when they can decide: approve or reject, show a warning or not, charge shipping or give it free. Python’s if statement runs a block of code only when a condition is true.

if#

temperature = 34

if temperature > 30:
    print("It's hot today.")
    print("Remember to drink water.")

print("Have a nice day.")

The condition is followed by a colon, and the block is indented. If the condition is false, the indented lines are skipped.

else and elif#

age = 16

if age >= 18:
    print("You can vote.")
else:
    print(f"You can vote in {18 - age} years.")

For more than two options, use elif (“else if”):

score = 76

if score >= 90:
    grade = "A"
elif score >= 75:
    grade = "B"
elif score >= 60:
    grade = "C"
elif score >= 40:
    grade = "D"
else:
    grade = "F"

print(f"Score {score}: grade {grade}")   # Score 76: grade B

Python checks each condition from the top and runs only the first block whose condition is true. That’s why the order matters: 95 is also >= 75, but it’s caught by the first condition.

Comparison operators#

Operator	Meaning
==	Equal
!=	Not equal
< >	Less / greater than
<= >=	Less / greater than or equal

Python lets you chain comparisons, just like in maths:

age = 25
if 18 <= age < 60:
    print("Working age")

and, or, not#

age = 22
has_ticket = True
is_vip = False

if age >= 18 and has_ticket:
    print("Welcome to the concert.")

if is_vip or age < 12:
    print("Free entry.")
else:
    print("Standard entry.")

if not has_ticket:
    print("Please buy a ticket.")

Python uses the words and, or and not instead of symbols. and is evaluated before or; use brackets to make mixed conditions clear.

Truthy checks#

Empty strings, empty collections, 0 and None count as false:

username = "   ".strip()
cart = []

if not username:
    print("Please enter a username.")

if cart:
    print(f"{len(cart)} items in cart")
else:
    print("Your cart is empty.")

This is the Pythonic way: write if cart: rather than if len(cart) > 0:.

Membership: in#

day = "sat"
if day in ("sat", "sun"):
    print("Weekend!")

email = "asha@example.com"
if "@" not in email:
    print("Invalid email")
else:
    print("Looks like an email")

in works with strings, lists, tuples, sets and dictionary keys, and replaces long chains of or.

Identity: is#

Use is to compare with None (and True/False in rare cases):

result = None
if result is None:
    print("No result yet.")

== asks “are these equal?”; is asks “is this the very same object?”. For None, always use is.

Nested conditions and guard clauses#

Nested ifs work but get hard to read:

def can_checkout(user, cart):
    if user is not None:
        if cart:
            if user.get("address"):
                return "OK"
            else:
                return "Add an address"
        else:
            return "Cart is empty"
    else:
        return "Please log in"

print(can_checkout({"address": "Pune"}, ["book"]))

The same logic with guard clauses (return early on problems):

def can_checkout(user, cart):
    if user is None:
        return "Please log in"
    if not cart:
        return "Cart is empty"
    if not user.get("address"):
        return "Add an address"
    return "OK"

print(can_checkout({"address": "Pune"}, []))
print(can_checkout(None, ["book"]))

Flat code is easier to read and extend. (You’ll learn def and return in the Functions lesson.)

Conditional expressions (one-line if)#

age = 20
status = "adult" if age >= 18 else "minor"
print(status)

items = 3
print(f"{items} item{'s' if items != 1 else ''}")

value_if_true if condition else value_if_false: handy for choosing between two values. Use a full if statement for anything longer.

match: choosing from many patterns (Python 3.10+)#

command = "stop"

match command:
    case "start":
        print("Starting…")
    case "stop" | "quit":
        print("Stopping.")
    case "help":
        print("Commands: start, stop, help")
    case _:
        print(f"Unknown command: {command}")

• case "stop" | "quit" matches either value.
• case _ matches anything else (like else).
• No break needed; only one case runs.

match can also unpack structures:

point = (0, 5)

match point:
    case (0, 0):
        print("Origin")
    case (0, y):
        print(f"On the y-axis at {y}")
    case (x, 0):
        print(f"On the x-axis at {x}")
    case (x, y):
        print(f"At {x}, {y}")

For simple value checks, if/elif is often clearer; match shines with many fixed options or structured data.

A practical example: shipping cost#

def shipping_cost(total, is_member, city):
    if is_member:
        return 0
    if total >= 500:
        return 0
    if city in {"Pune", "Mumbai"}:
        return 30
    return 60

print(shipping_cost(350, False, "Pune"))    # 30
print(shipping_cost(350, False, "Delhi"))   # 60
print(shipping_cost(800, False, "Delhi"))   # 0
print(shipping_cost(100, True, "Delhi"))    # 0

Common mistakes#

• **= instead of ==** in a condition: if x = 5: is a SyntaxError in Python (good, it catches the bug).
• Missing colon or wrong indentation.
• Wrong order of elif branches.
• **if x == "a" or "b":** is always true; write if x in ("a", "b"):.
• **== None** instead of is None.

Exercise#

1. Ask for a number (or set one) and print whether it’s positive, negative or zero.
2. Write a BMI category checker: under 18.5, 18.5–24.9, 25–29.9, 30 and above.
3. Given an hour (0–23), print “Good morning”, “Good afternoon” or “Good evening”.
4. Check if a year is a leap year: divisible by 4, except centuries, unless divisible by 400.
5. Write a match statement for a simple calculator that handles +, -, * and /.

Summary#

• if, elif and else run different blocks based on conditions; the first true branch wins.
• Combine conditions with and, or and not; chain comparisons like 0 < x < 10.
• Use in for membership, is None for None, and truthy checks for empty values.
• Conditional expressions and match handle short choices and many fixed options.

Next, you’ll repeat actions with for and while loops.