Python Basics: Complete Revision Notes 9th-AI

Introduction

1. Python File Extension

Python programs are saved with the .py extension. When you create a Python file, always name it something like program.py or calculator.py.

Remember:

• Correct: myprogram.py
• Incorrect: myprogram.python, myprogram.pt

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2. Operators and Order of Operations

Arithmetic Operators

Python follows the PEMDAS/BODMAS rule for order of operations:

• Parentheses/Brackets – ()
• Exponentiation – **
• Multiplication & Division – *, /, //, %
• Addition & Subtraction – +, –

Example:

print(10 + 5 * 2)  # Output: 20
# Multiplication happens first: 5 * 2 = 10
# Then addition: 10 + 10 = 20

Important Operators

Operator	Purpose	Example
+	Addition	5 + 3 = 8
-	Subtraction	5 - 3 = 2
*	Multiplication	5 * 3 = 15
/	Division (with decimal)	5 / 2 = 2.5
//	Floor Division (integer)	5 // 2 = 2
%	Modulus (remainder)	5 % 2 = 1
**	Exponentiation (power)	5 ** 2 = 25

Key Difference: / vs //

• / (Division): Gives the complete answer with decimals print(7 / 2) # Output: 3.5
• // (Floor Division): Gives only the whole number part print(7 // 2) # Output: 3

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3. Data Types in Python

Boolean Data Type (bool)

The bool data type stores only two values:

• True
• False

Example:

is_student = True
is_adult = False
print(5 > 3)  # Output: True

Common Data Types Summary

Data Type	Description	Example
int	Whole numbers	10, -5, 0
float	Decimal numbers	3.14, -0.5
str	Text/String	"Hello", 'Python'
bool	True/False	True, False

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4. Essential Python Functions

The print() Function

Used to display output on the screen.

print("Hello World")
print(10 + 5)
print("Sum:", 15)

Variables

A variable is a named memory location that stores data.

name = "Rahul"
age = 15
marks = 95.5
is_passed = True

Rules for Variable Names:

• Can contain letters, numbers, and underscores
• Must start with a letter or underscore
• Case-sensitive (Name and name are different)
• No spaces allowed

Type Conversion Functions

int() – Converts to integer

num = int("25")  # Converts string "25" to integer 25
x = int(3.9)     # Converts float 3.9 to integer 3

Other conversion functions:

• float() – Converts to decimal number
• str() – Converts to string

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5. Python Syntax Rules

Case Sensitivity

Python is case-sensitive, meaning uppercase and lowercase letters are treated differently.

Name = "John"
name = "Jane"
print(Name)  # Output: John
print(name)  # Output: Jane

Comments

The # symbol is used for single-line comments.

# This is a comment
print("Hello")  # This is also a comment

Indentation

Indentation (spaces at the beginning of a line) is used to define a block of code in Python.

if age >= 18:
    print("You can vote")  # This line is indented
    print("Welcome!")      # This line is also indented

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6. Taking User Input

Use the input() function to get input from users:

name = input("Enter your name: ")
age = int(input("Enter your age: "))  # Convert to integer

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7. Sample Programs You Must Practice

Program 1: Sum of Two Numbers

# Taking input from user
num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))

# Calculating sum
sum = num1 + num2

# Displaying output
print("Sum =", sum)

Program 2: Simple Interest Calculator

# Input
principal = float(input("Enter principal amount: "))
rate = float(input("Enter rate of interest: "))
time = float(input("Enter time period (years): "))

# Calculate Simple Interest
simple_interest = (principal * rate * time) / 100

# Output
print("Simple Interest =", simple_interest)

Formula: SI = (P × R × T) / 100

Program 3: Check Even or Odd Number

# Input
number = int(input("Enter a number: "))

# Check if even or odd
if number % 2 == 0:
    print(number, "is Even")
else:
    print(number, "is Odd")

Logic: If a number is divisible by 2 (remainder is 0), it’s even. Otherwise, it’s odd.

Program 4: Voting Eligibility Checker

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

# Check eligibility using simple if statement
if age >= 18:
    print("You are eligible to vote")
if age < 18:
    print("You are not eligible to vote")

Note: Voting age in India is 18 years.

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Quick Revision Checklist

make sure you can:

✅ Write the correct Python file extension (.py)
✅ Calculate expressions using BODMAS rule
✅ Identify data types (int, float, str, bool)
✅ Know all arithmetic operators, especially ** and //
✅ Use print() to display output
✅ Create and use variables
✅ Convert strings to integers using int()
✅ Write comments using #
✅ Use proper indentation in if statements
✅ Take input from users using input()
✅ Write programs for: sum, simple interest, even/odd, voting eligibility

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Common Mistakes to Avoid

1. Forgetting to convert input(): input() always returns a string, so convert it to int or float when needed.
2. Incorrect operator: Use ** for power, not ^
3. Wrong division operator: Remember the difference between / and //
4. Indentation errors: Always indent code inside if statements properly
5. Case sensitivity: print and Print are different in Python

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Practice Tips

1. Write code by hand – Practice writing programs on paper
2. Run programs – Type and execute each program on your computer
3. Modify examples – Change values and see what happens
4. Solve previous papers – Practice similar questions
5. Understand logic – Don’t just memorize, understand why the code works

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