One of the most powerful features of programming is the ability to make decisions based on different situations. In Python, conditional statements allow programs to execute different blocks of code depending on whether a condition is true or false.

From login authentication and ATM systems to AI-powered applications and e-commerce platforms, decision-making is an essential part of modern software development. Python provides simple yet powerful conditional statements such as if, elif, and else to control program flow.

In this lesson, you’ll learn how to create decision-making logic using conditional statements, combine multiple conditions with logical operators, write nested conditions, and solve real-world programming problems through practical examples.

By the end of this lesson, you’ll be able to build intelligent programs that respond dynamically to user input and different scenarios.

Learning Objectives

After completing this lesson, you will be able to:

• Understand conditional statements.
• Use the if statement.
• Use if...else.
• Work with if...elif...else.
• Create nested conditions.
• Combine conditions using logical operators.
• Write decision-making programs.
• Avoid common mistakes in conditional programming.

Topics Covered

• Introduction to Conditional Statements
• Boolean Expressions
• The if Statement
• The if...else Statement
• The if...elif...else Statement
• Nested if Statements
• Logical Operators with Conditions
• Real-World Examples
• Best Practices

Detailed Lesson Content

Introduction to Conditional Statements

Imagine you are using an ATM. The machine first checks:

• Is the PIN correct?
• Does the account have enough balance?
• Is the withdrawal amount within the daily limit?

Depending on these conditions, the ATM either allows the transaction or displays an error message.

Similarly, online shopping websites check whether:

• A product is in stock.
• A coupon is valid.
• The customer qualifies for free shipping.
• Payment has been completed successfully.

These decisions are made using conditional statements.

A conditional statement evaluates a condition and executes different blocks of code based on whether the condition is True or False.

What is a Condition?

A condition is an expression that returns either:

• True
• False

Example:

age = 20

print(age >= 18)

Output

True

Python uses these Boolean values to decide which code should execute.

The if Statement

The simplest conditional statement is the if statement.

Syntax

if condition:
    # Code to execute

Example:

age = 20

if age >= 18:
    print("You are eligible to vote.")

Output

You are eligible to vote.

If the condition is False, nothing happens.

Understanding Indentation

Python uses indentation (spaces) instead of braces {}.

Correct:

marks = 80

if marks >= 50:
    print("Pass")

Incorrect:

if marks >= 50:
print("Pass")

Python will generate an IndentationError.

The if…else Statement

Sometimes we need two possible outcomes.

Example:

age = 16

if age >= 18:
    print("Eligible")
else:
    print("Not Eligible")

Output

Not Eligible

Flow:

• If condition is True → execute if
• Otherwise → execute else

The if…elif…else Statement

When multiple conditions need to be checked, use elif.

Example:

marks = 82

if marks >= 90:
    print("Grade A+")

elif marks >= 75:
    print("Grade A")

elif marks >= 60:
    print("Grade B")

else:
    print("Grade C")

Output

Grade A

Python checks conditions from top to bottom and stops when the first True condition is found.

Nested if Statements

An if statement can exist inside another if statement.

Example:

age = 22
citizen = True

if age >= 18:

    if citizen:
        print("Eligible to Vote")

    else:
        print("Citizen Verification Required")

else:
    print("Not Eligible")

Output

Eligible to Vote

Nested conditions are useful when multiple checks depend on one another.

Using Logical Operators

Logical operators allow us to combine multiple conditions.

AND Operator

Both conditions must be True.

age = 25
experience = 3

if age >= 21 and experience >= 2:
    print("Eligible")

OR Operator

Only one condition needs to be True.

marks = 92

if marks >= 90 or marks >= 85:
    print("Scholarship Eligible")

NOT Operator

Reverses the result.

is_blocked = False

if not is_blocked:
    print("Login Successful")

Real-World Example 1: Login System

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

if username == "admin" and password == "12345":
    print("Login Successful")

else:
    print("Invalid Username or Password")

Real-World Example 2: ATM Withdrawal

balance = 25000
withdraw = int(input("Enter Amount: "))

if withdraw <= balance:

    balance = balance - withdraw

    print("Transaction Successful")
    print("Remaining Balance:", balance)

else:

    print("Insufficient Balance")

Real-World Example 3: Student Grading System

marks = int(input("Enter Marks: "))

if marks >= 90:

    print("Grade A+")

elif marks >= 80:

    print("Grade A")

elif marks >= 70:

    print("Grade B")

elif marks >= 60:

    print("Grade C")

else:

    print("Fail")

Real-World Example 4: E-commerce Discount

bill = float(input("Enter Bill Amount: "))

if bill >= 5000:

    discount = bill * 0.20

elif bill >= 3000:

    discount = bill * 0.10

else:

    discount = 0

final_bill = bill - discount

print("Discount:", discount)

print("Final Bill:", final_bill)

Common Mistakes

Forgetting the Colon

Incorrect

if age >= 18

Correct

if age >= 18:

Incorrect Indentation

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

Correct

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

Using = Instead of ==

Incorrect

if age = 18:

Correct

if age == 18:

Best Practices

• Write simple and readable conditions.
• Avoid deeply nested conditions whenever possible.
• Use meaningful variable names.
• Use logical operators efficiently.
• Test all possible outcomes.
• Handle invalid user input gracefully.

Key Takeaways

• Conditional statements control program flow.
• if executes code when a condition is True.
• else handles alternative scenarios.
• elif allows multiple conditions.
• Nested if statements support complex decision-making.
• Logical operators combine multiple conditions.
• Proper indentation is mandatory in Python.