Python if Statements

What Is Decision Making?

Programs often need to make decisions.

For example:

• If a student scores 50 or more, they pass.
• If a customer's cart total is high enough, they receive free delivery.
• If a user's password is correct, access is granted.
• If the temperature is high, the program can display a warning.

Python uses conditional statements to control which parts of a program should execute.

The if Statement

The simplest conditional statement in Python is if.

if condition:
    # code to execute

Example:

age = 20

if age >= 18:
    print("You are an adult.")

You are an adult.

Since age >= 18 is true, Python executes the indented statement.

Understanding Conditions

A condition is an expression that produces either True or False.

age = 25

print(age >= 18)
print(age < 18)

True
False

These Boolean results can be used directly inside conditional statements.

age = 25

if age >= 18:
    print("Eligible")

Indentation & Code Blocks

Python uses indentation to identify the code that belongs to an if statement.

age = 20

if age >= 18:
    print("Adult")
    print("Eligible to vote")

Both print() statements belong to the if block because they are indented.

The standard Python convention is four spaces per indentation level.

if with Comparison Operators

Comparison operators are frequently used to create conditions.

score = 85

if score >= 50:
    print("Passed")

Common comparison operators include:

• == — equal to
• != — not equal to
• > — greater than
• < — less than
• >= — greater than or equal to
• <= — less than or equal to

temperature = 35

if temperature > 30:
    print("It is hot today.")

It is hot today.

if with Boolean Values

Boolean variables can be used directly as conditions.

is_logged_in = True

if is_logged_in:
    print("Welcome back!")

Welcome back!

You usually do not need to write if is_logged_in == True:. The simpler form is clearer.

# Preferred
if is_logged_in:
    print("Welcome")

# Less concise
if is_logged_in == True:
    print("Welcome")

The else Statement

An if statement handles the true condition. The else block handles the alternative.

age = 16

if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")

You are a minor.

Only one of these two branches executes.

The elif Statement

Use elif, short for "else if", when you have multiple possible conditions.

marks = 78

if marks >= 90:
    print("Grade A+")
elif marks >= 80:
    print("Grade A")
elif marks >= 70:
    print("Grade B")
else:
    print("Needs improvement")

Grade B

Python checks conditions from top to bottom. Once one condition is true, its block runs and the remaining branches are skipped.

Multiple Conditions

You can use several elif branches when a program has multiple possible outcomes.

age = 35

if age < 13:
    category = "Child"
elif age < 20:
    category = "Teenager"
elif age < 60:
    category = "Adult"
else:
    category = "Senior"

print(category)

Adult

Logical Operators with if

You can combine conditions using and, or, and not.

Using and

age = 25
has_id = True

if age >= 18 and has_id:
    print("Access granted")

Using or

is_admin = False
is_manager = True

if is_admin or is_manager:
    print("You can access this section.")

Using not

is_blocked = False

if not is_blocked:
    print("Account is active.")

Nested if Statements

An if statement can exist inside another if statement.

age = 25
has_ticket = True

if age >= 18:

    if has_ticket:
        print("You may enter.")
    else:
        print("You need a ticket.")

else:
    print("You must be an adult.")

You may enter.

Nested conditions are useful when the second decision only matters after the first condition has been satisfied.

Membership Conditions

The in and not in operators can be used directly inside conditions.

available_colors = ["red", "blue", "green"]

color = "blue"

if color in available_colors:
    print("Color is available.")
else:
    print("Color is not available.")

Color is available.

This technique is useful for checking membership in lists, sets, strings, and dictionary keys.

Identity Conditions

Python's is and is not operators check object identity.

They are particularly useful when checking for None.

value = None

if value is None:
    print("No value was provided.")

No value was provided.

Conditions with Strings

Strings can be compared inside conditional statements.

username = "admin"

if username == "admin":
    print("Welcome, administrator.")
else:
    print("Welcome, user.")

Welcome, administrator.

String comparisons are case-sensitive.

password = "Python123"

if password == "python123":
    print("Correct password.")
else:
    print("Incorrect password.")

Incorrect password.

Conditions with Numbers

Numeric conditions are common in calculations, applications, and business logic.

cart_total = 6500

if cart_total >= 5000:
    print("You qualify for free delivery.")
else:
    print("Delivery charges apply.")

You qualify for free delivery.

Another Example

stock = 4

if stock == 0:
    print("Out of stock.")
elif stock < 5:
    print("Low stock.")
else:
    print("In stock.")

Low stock.

if Statements with User Input

Conditional logic becomes much more useful when combined with user input.

age = int(input("Enter your age: "))

if age >= 18:
    print("You are eligible.")
else:
    print("You are not eligible.")

Remember that input() returns a string, so the value is converted to an integer using int().

Login Example

username = input("Username: ")
password = input("Password: ")

if username == "admin" and password == "1234":
    print("Login successful.")
else:
    print("Invalid credentials.")

Real-World Examples

Student Result System

marks = 82

if marks >= 90:
    grade = "A+"
elif marks >= 80:
    grade = "A"
elif marks >= 70:
    grade = "B"
elif marks >= 60:
    grade = "C"
elif marks >= 50:
    grade = "D"
else:
    grade = "F"

print(f"Grade: {grade}")

Grade: A

Discount Calculator

total = 12000

if total >= 10000:
    discount = 20
elif total >= 5000:
    discount = 10
else:
    discount = 0

print(f"Discount: {discount}%")

Discount: 20%

Weather Alert

temperature = 42

if temperature >= 40:
    print("Extreme heat warning.")
elif temperature >= 30:
    print("Hot weather.")
elif temperature >= 20:
    print("Pleasant weather.")
else:
    print("Cool weather.")

Extreme heat warning.

Common Mistakes

Mistake 1 — Using = Instead of ==

# Incorrect
if age = 18:
    print("Adult")

The single = operator assigns a value. Comparison requires ==.

# Correct
if age == 18:
    print("Exactly 18")

Mistake 2 — Forgetting the Colon

# Incorrect
if age >= 18
    print("Adult")

# Correct
if age >= 18:
    print("Adult")

Mistake 3 — Incorrect Indentation

# Incorrect
if age >= 18:
print("Adult")

# Correct
if age >= 18:
    print("Adult")

Mistake 4 — Wrong Condition Order

marks = 95

if marks >= 50:
    print("Pass")
elif marks >= 90:
    print("Excellent")

The second condition is never reached for 95 because the first condition is already true.

More specific conditions should generally come before broader conditions.

marks = 95

if marks >= 90:
    print("Excellent")
elif marks >= 50:
    print("Pass")

Practice Exercises

Mini Challenge — Student Result Analyzer

Build a complete student result program using conditional statements.

The program should:

• Ask the user for their name.
• Ask for marks in three subjects.
• Calculate the total and average.
• Determine whether the student passed or failed.
• Assign a grade based on the average.
• Display a professional result summary.

name = input("Enter student name: ")

math = float(input("Math marks: "))
english = float(input("English marks: "))
computer = float(input("Computer marks: "))

total = math + english + computer
average = total / 3

if average >= 90:
    grade = "A+"
elif average >= 80:
    grade = "A"
elif average >= 70:
    grade = "B"
elif average >= 60:
    grade = "C"
elif average >= 50:
    grade = "D"
else:
    grade = "F"

if average >= 50:
    result = "Passed"
else:
    result = "Failed"

print("\n--- Student Result ---")
print(f"Name: {name}")
print(f"Total: {total:.2f}")
print(f"Average: {average:.2f}")
print(f"Grade: {grade}")
print(f"Result: {result}")

--- Student Result ---
Name: Ali
Total: 246.00
Average: 82.00
Grade: A
Result: Passed

Quick Knowledge Check

Test your understanding of Python conditional statements.

Lesson Summary

Conditional statements allow Python programs to make decisions and choose which code should execute.