What You'll Learn

This page covers the core fundamentals of Python, step by step.

1. Program Structure — how a Python program is written
2. Variables & Data Types — storing and labeling data
3. Input/Output — taking input and printing output
4. Operators — performing calculations and comparisons
5. Conditional Statements — making decisions in code
6. Loops — repeating actions efficiently
7. Functions — organizing code into reusable blocks
8. Lists — storing multiple values together
9. Quiz — test what you've learned

Program Structure

Python programs don't need a special starting function like main() in C — code just runs from top to bottom, line by line.A Python program usually contains comments, variables, input/output statements, functions, and the main program code.

Here's the simplest possible Python program:

print("Hello, World!")
# Output: Hello, World!

That's it — one line. No #include, no curly braces, no semicolons. Python relies on indentation (spacing) instead of symbols to organize code, which is a big part of why it's considered so easy to read.

A slightly bigger example

name = "Aravind"
print("Hello,", name)
# Output: Hello, Aravind

Here,nameis a variable storing the name "Aravind",and print() is a built-in function used to display output on the screen.

Variables & Data Types

A variable is a name used to store a value in a Python program. The value of a variable can be changed during the program.

Declaring a Variable in Python

age = 21
price = 99.50
grade = 'A'
is_passed = True

Common Data Types

• int — whole numbers, like 10 or -5
• float — decimal numbers, like 3.14
• str — text, wrapped in quotes, like "hello"
• bool — either True or False

Checking a variable's type

Since Python doesn't require you to state the type, you can check it anytime using type().

age = 21
print(type(age))
# Output: <class 'int'>

Naming Rules for Variables

• It must start with a letter or underscore, not a number
• It can contain letters, numbers, and underscores
• No spaces allowed
• They are case-sensitive — age and Age are different variables

Example

name = "Akshay"
age = 18
print(name, "is", age, "years old")
# Output: Akshay is 18 years old

Comments

A comment is a note in your code that Python completely ignores when running the program. It's just for humans — to explain what the code does, leave reminders, or temporarily disable a line without deleting it.

Single-line comments

Use # — everything after it on that line is ignored.

# This prints a greeting
print("Hello, World!")
# Output: Hello, World!

Multi-line comments

Python doesn't have a true built-in multi-line comment symbol, but the common trick is to use triple quotes.

"""
This is a multi-line comment.
It can span several lines.
"""
print("Hello, World!")
# Output: Hello, World!

Why comments matter

• Helps you (or someone else) understand the code later
• Useful for explaining tricky logic
• Can temporarily turn off a line while testing

Input/Output

Input is the process of taking data from the user while a Python program is running, Output is the process of displaying information or results to the user.

Python uses print() to show output and input() to take input from the user.

Output with print()

print("Hello, World!")
Hello, World!

Input with input()

input() always returns text (a string), even if the user types a number — so if you need it as a number, you have to convert it.

name = input("Enter your name: ")
print("Hello,", name)

"""Output:
  Enter your name: Akshay 
  Hello, Akshay"""

Converting input to a number(Type casting)

age = input("Enter your age: ")
age = int(age)
print("You are", age, "years old")
# Output: Enter your age: 21
# You are 21 years old

int() converts text into a whole number. Use float() instead if you're expecting a decimal.

Full Example

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

print(name, "is", age, "years old")
# Output: Enter your name: Akshay
# Enter your age: 21
# Akshay is 21 years old

Key Points

• input() always returns a string, even for numbers
• Use int() or float() to convert input when doing math with it
• print() can display multiple values separated by commas

Operators

Operators are symbols used to perform operations on values and variables.

Main types of operators:

• Arithmetic operators – Perform mathematical calculations.
• Assignment operators – Assign values to variables.
• Comparison operators – Compare two values.
• Logical operators – Used to combine conditions.
• Bitwise operators – Used to perform operations on binary values.

Arithmetic Operators

• + — addition
• - — subtraction
• * — multiplication
• / — division (always gives a decimal result)
• // — floor division (drops the decimal part)
• % — modulus (remainder after division)
• ** — exponent (power)

print(10 / 3)   # 3.333...
print(10 // 3)  # 3
print(10 % 3)   # 1
print(2 ** 3)   # 8

Assignment Operators

• = — Assigns a value to a variable
• += — Adds a value to the variable and stores the result.
• -= — Subtracts a value from the variable and stores the result.
• *= — Multiplies the variable by a value and stores the result.
• /= — Divides the variable by a value and stores the result.
• %= — Finds the remainder and stores it in the variable.

a = 10

a += 5    # a = 15
a -= 3    # a = 12
a *= 2    # a = 24
    

Comparison Operators

• == — Checks if two values are equal.
• != — Checks if two values are not equal.
• > — Checks if the first value is greater.
• < — Checks if the first value is smaller.
• >= — Checks if the first value is greater than or equal.
• <= — Checks if the first value is smaller than or equal.

Logical Operators

Python spells these out as words instead of symbols, unlike C/C++.

• and — True if both conditions are true
• or — True if at least one condition is true
• not — Reverses the result (True becomes False and vice versa).

age = 20
has_id = True

print(age >= 18 and has_id) 
# True

Try this :

Conditional Statements

Conditional statements are used to make decisions in a Python program based on whether a condition is True or False. Python doesn't use curly braces like C/C++; instead, it uses indentation to show which lines belong inside the condition.

The if statement

The if statement executes a block of code only when the given condition is true.

age = 18

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

Explanation:

Here, age >= 18 is checked. Since the condition is true, "Adult" is printed.

Adding else

The if-else statement executes one block when the condition is true and another block when it is false.

age = 15

if age >= 18:
    print("Adult")
else:
    print("Minor")
# Output: Minor

Explanation:

Since age >= 18 is false, the else block is executed. Therefore, "Minor" is printed.

if-elif-else Statement

The if-else statement executes one block when the condition is true and another block when it is false.

mark = 75

if mark >= 90:
    print("Grade: A")
elif mark >= 75:
    print("Grade: B")
else:
    print("Grade: C")
# Output: Grade: B

Explanation:

Python first checks the if condition. If it is false, it checks elif; here, 75 >= 75 is true, so "B" is printed.

Nested if Statement

A nested if means having an if statement inside another if statement.

age = 20

if age >= 18:
    if age >= 21:
        print("Eligible")
# Output: (nothing is printed)

First, Python checks whether the age is at least 18. If true, it checks the second condition (age >= 21). Here, the second condition is false, so nothing is printed.

Important: Indentation matters

In Python, indentation isn't just for readability — it's part of the syntax. If the spacing is wrong, the program will throw an error. Always use consistent spacing (usually 4 spaces) under an if, elif, or else.

Loops

Loops are used to repeat a block of code multiple times until a particular condition is met.

Loops allow the same block of code to be executed multiple times without writing it repeatedly.

The for loop

A for loop is used to iterate over a sequence such as a list, string, or range.

Best used when you want to repeat something a specific number of times, often using range().

for i in range(1, 6):
print(i)
# Output: 1 2 3 4 5

Explanation:

• The for loop repeats the code for each value in the given range.
• Here, i takes values from 1 to 5, and each value is printed.

The while loop

A while loop repeatedly executes a block of code as long as the condition is true. Useful when you don't know in advance how many times it'll repeat.

i = 1
while i <= 5:
    print(i)
    i += 1
# Output: 1 2 3 4 5

Explanation:

• The loop continues while i is less than or equal to 5.
• i += 1 increases the value after each iteration.

Loop control statements

• break — stops the loop completely
• continue — skips the current round and moves to the next one

for i in range(1, 6):
    if i == 3:
        continue
    print(i)
# prints: 1 2 4 5 (skips 3)

Key Points

• Python doesn't have a do-while loop like C/C++
• range(start, stop) stops one number before the given stop value
• Indentation defines what's inside the loop, just like with if statements

Functions

A function is a reusable block of code that performs a specific task. Instead of writing the same instructions repeatedly, you define them once and just call the function whenever you need it.

Defining a function

Use the def keyword, followed by a name and parentheses.

def greet():
    print("Hello there!")

greet()
# Output: Hello there!

Functions with parameters

Parameters let you pass information into a function so it can work with different values each time.

def add(a, b):
    return a + b

result = add(5, 3)
print(result)
# Output: 8

Breaking it down

• def — keyword used to define a function
• add(a, b) — the function name and the inputs it accepts
• return a + b — sends the result back to wherever the function was called

Functions without a return value

Some functions just perform an action (like printing something) without needing to send anything back.

def show_message():
    print("This function doesn't return anything.")

show_message()
# Output: This function doesn't return anything.

Default parameter values

You can give a parameter a default value, used automatically if no value is passed in.

def greet(name="friend"):
    print("Hello,", name)

greet()
greet("Akshay")
# Output: Hello, friend
# Hello, Akshay

Why use functions

• Avoids repeating the same code multiple times
• Makes programs easier to read and organize
• Makes it easier to fix bugs, since related code lives in one place

Lists

A list is a collection used to store multiple values in a single variable. Lists are ordered, changeable, and allow duplicate values.

Creating a list

numbers = [10, 20, 30, 40, 50]
print(numbers)
# Output: [10, 20, 30, 40, 50]

Here, fruits is a list containing three items. The items are stored in order and can be changed later.

Accessing elements

Each item has a position called an index, starting from 0, just like arrays.

numbers = [10, 20, 30, 40, 50]
print(numbers[0])
print(numbers[2])
# Output: 10
# 30

Changing a value

numbers = [10, 20, 30, 40, 50]
    
numbers[1] = 99
print(numbers)
# Output: [10, 99, 30, 40, 50]

Adding items

Unlike arrays, Python lists can grow — by using append() to add a new item to the end.

numbers.append(60)
print(numbers)
# Output: [10, 99, 30, 40, 50, 60]

Removing items

numbers = [10, 99, 30, 40, 50]
numbers.remove(99)
print(numbers)
# Output: [10, 30, 40, 50, 60]

Looping through a list

for num in numbers:
print(num)
# Output: 10
# 30
# 40
# 50
# 60

Key Points

• Lists can hold mixed data types (numbers, text, etc. together)
• Lists can grow or shrink using append() and remove()
• Indexing starts at 0, same as arrays