Python If…Else: Conditions, elif and Nested If

Python with AI Tutorial · Chapter 17 of 48

A Python if else statement lets your program make decisions. Python evaluates a condition, runs one block of code when it is True and a different block when it is False. With elif you can test several conditions in order, and by nesting one if inside another you can model more detailed business rules.

The if statement

An if statement starts with the keyword if, a condition and a colon. The indented lines beneath it only run when the condition is true.

invoice_total = 1250
if invoice_total > 1000:
    print("Large invoice - needs manager approval")
print("Invoice processed")

Output:
Large invoice - needs manager approval
Invoice processed

The last print() is not indented, so it runs regardless of the condition. Indentation is how Python knows which lines belong to the if block.

Comparison operators

Conditions are usually built with comparison operators. Each one returns a Boolean value, True or False.

Strings can be compared as well. "apple" < "banana" is True because Python compares character by character in Unicode order, which also means uppercase letters sort before lowercase ones. When you compare user-entered text, normalise it with .lower() or .strip() first so that “Paid” and “paid ” are treated the same.

if…else

Add an else block to run code when the condition is false. Exactly one of the two blocks will execute.

stock = 4
reorder_level = 10
if stock < reorder_level:
    print("Reorder now")
else:
    print("Stock is fine")

Output:
Reorder now

elif for several conditions

elif is short for “else if”. Python checks each condition from top to bottom and runs the first block that matches, then skips the rest.

monthly_sales = 48000
if monthly_sales >= 60000:
    bonus = 0.10
elif monthly_sales >= 40000:
    bonus = 0.05
elif monthly_sales >= 20000:
    bonus = 0.02
else:
    bonus = 0
print(f"Bonus rate: {bonus:.0%}")
print(f"Bonus amount: {monthly_sales * bonus:,.2f}")

Output:
Bonus rate: 5%
Bonus amount: 2,400.00

Order matters. If the >= 20000 test came first, a salesperson with 48,000 in sales would receive the 2% rate because that condition is also true.

Write a Python function commission_rate(sales) using if, elif and else. Rates: 0% below 10,000; 3% from 10,000 to 24,999; 5% from 25,000 to 49,999; 8% at 50,000 or above. Then show what goes wrong if the conditions are written in the opposite order.

Combining conditions with and, or, not

Logical operators let one if check several things at once. and needs every part to be true, or needs at least one and not flips the result.

department = "Finance"
years_of_service = 6
is_contractor = False

if department == "Finance" and years_of_service >= 5:
    print("Eligible for the leadership programme")
if department == "Sales" or department == "Marketing":
    print("Attends the quarterly revenue meeting")
if not is_contractor:
    print("Receives the annual bonus")

Output:
Eligible for the leadership programme
Receives the annual bonus

Python evaluates these operators lazily, a behaviour called short-circuiting. With and, if the first part is False the second part is never checked; with or, a True first part ends the test. That lets you write safe checks such as if customer and customer.get("email"):, where the second test only runs when customer is not empty.

Nested if statements

An if can sit inside another if. The inner test only runs when the outer condition passed, which is handy for rules that depend on an earlier decision.

order_value = 850
customer_type = "wholesale"

if order_value > 500:
    if customer_type == "wholesale":
        discount = 0.15
    else:
        discount = 0.10
else:
    discount = 0
print(f"Discount: {discount:.0%}, pay {order_value * (1 - discount):.2f}")

Output:
Discount: 15%, pay 722.50

Two levels of nesting are usually fine. Deeper than that, consider rewriting with and or moving the logic into a function.

Truthy and falsy values

Any object can be used as a condition. Empty containers, zero and None count as false; almost everything else counts as true. This makes checks like “is the list empty?” very short.

overdue_invoices = []
if overdue_invoices:
    print(f"{len(overdue_invoices)} invoices overdue")
else:
    print("No overdue invoices")

Output:
No overdue invoices

A related idiom is checking for None, which Python uses to mean “no value yet”. Write if manager is None: rather than == None; the is operator tests identity and is the form recommended by the official style guide. The same applies to if result is not None:.

Short hand if and the conditional expression

A single-line if is allowed, and Python also has a conditional expression (often called a ternary) that picks one of two values.

temperature = 31
if temperature > 30: print("Warehouse cooling on")

balance = 120.0
label = "Paid" if balance == 0 else "Unpaid"
print(label)

Output:
Warehouse cooling on
Unpaid

Use the conditional expression for simple value choices only. If each branch needs several statements, write a normal if...else block.

The pass statement

An if block cannot be empty. When you have not written the logic yet, use pass as a placeholder so the file still runs.

region = "EMEA"
if region == "EMEA":
    pass  # tax rules to be added
print("Done")

Output:
Done

Here is a nested Python if statement that decides shipping cost from order_value and country. Rewrite it with a flat if/elif chain using "and", keep the same behaviour, and give me a table of 6 test inputs with the expected output for both versions.

Common mistakes

• Forgetting the colon at the end of the if, elif or else line.
• Mixing tabs and spaces in the indented block, which causes an IndentationError.
• Comparing a number to a string, for example input() returns text, so "5" > 3 raises a TypeError.
• Writing elif conditions in the wrong order so a broader test hides a narrower one.
• Using == to compare with None; the idiomatic form is is None.

Exercise

Write a program that stores an employee’s hours_worked (a number) and prints “Overtime” if it is above 40, “Full time” if it is exactly 40, “Part time” if it is at least 20, and “Casual” otherwise. Test it with 45, 40, 25 and 10.

for hours_worked in (45, 40, 25, 10):
    if hours_worked > 40:
        category = "Overtime"
    elif hours_worked == 40:
        category = "Full time"
    elif hours_worked >= 20:
        category = "Part time"
    else:
        category = "Casual"
    print(hours_worked, category)

Output:
45 Overtime
40 Full time
25 Part time
10 Casual

Related chapters

• Python Booleans
• Python Operators
• Python While Loops
• Python with AI course hub

FAQ

What is the difference between if, elif and else in Python?

if tests the first condition, elif tests further conditions only when the earlier ones were false, and else runs when none of the conditions matched. Only one block in the chain executes.

Does Python have a switch or case statement?

Python 3.10 added match...case for structural pattern matching, but for simple value checks an if...elif chain or a dictionary lookup is still the most common approach.

Can I write an if statement on one line in Python?

Yes. if x > 5: print(x) is valid, and the conditional expression a if condition else b chooses between two values on one line.

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