Lists in Python

Lists are one of the most powerful and commonly used data structures in Python. A list allows you to store multiple values in a single variable. Lists are flexible, dynamic, and can hold different types of data together.

In real-world applications, lists are used everywhere — storing user data, processing records, holding database rows, handling API responses, or organizing items in an application.

What Is a List?

A list is an ordered collection of items enclosed in square brackets [ ]. Each item has an index starting from 0. Lists can store numbers, strings, floats, booleans, or even other lists.

my_list = [10, "dataplexa", 3.14, True]

This list contains different types of values — Python allows this flexibility.

Why Lists Are Important?

Lists make your programs efficient because you can store and process large amounts of related data easily. They are widely used because:

• They are dynamic (you can add/remove items anytime)
• They maintain order
• They support slicing and indexing
• They can hold different data types together
• They are extremely fast for read/write operations

Creating Lists

Lists can be created in different ways:

numbers = [1, 2, 3, 4]
names = ["ram", "shyam", "dataplexa"]
mixed = [10, "hello", 5.5, False]
empty = []

Accessing List Elements (Indexing)

You can access individual elements using their index positions. Indexing starts from 0.

items = ["apple", "banana", "mango"]

print(items[0])   # apple
print(items[2])   # mango
print(items[-1])  # last item: mango

List Slicing

Slicing helps you extract a part of a list. This is useful when working with large data sets.

numbers = [10, 20, 30, 40, 50]

print(numbers[1:4])    # [20, 30, 40]
print(numbers[:3])     # [10, 20, 30]
print(numbers[2:])     # [30, 40, 50]

Modifying Lists

Lists are mutable, meaning you can change their values anytime.

items = ["apple", "banana", "mango"]

items[1] = "orange"
print(items)   # ["apple", "orange", "mango"]

Adding Elements to a List

1. append() – Adds item at the end

items = [1, 2, 3]
items.append(4)
print(items)

2. insert() – Adds item at a specific index

items = ["a", "b", "c"]
items.insert(1, "new")
print(items)

3. extend() – Adds all items from another list

a = [1, 2]
b = [3, 4, 5]

a.extend(b)
print(a)

Removing Elements from a List

1. remove() – Removes the first matching value

items = [10, 20, 30, 20]
items.remove(20)
print(items)

2. pop() – Removes item by index

items = ["x", "y", "z"]
items.pop(1)
print(items)

3. clear() – Removes all items

items = [1, 2, 3]
items.clear()
print(items)

Useful List Methods

Sorting Lists

Sorting is useful for organizing data alphabetically or numerically.

numbers = [40, 10, 20, 5]
numbers.sort()
print(numbers)

Looping Through a List

You can iterate through lists using a loop.

items = ["pen", "book", "laptop"]

for i in items:
    print(i)

List Comprehension (Advanced Feature)

List comprehension is a shorter and more efficient way to create lists.

squares = [x*x for x in range(1, 6)]
print(squares)

Real-World Example: Shopping Cart System

cart = []
cart.append("Phone")
cart.append("Charger")
cart.append("Earbuds")

print("Your Cart Items:")
for item in cart:
    print("-", item)

Real-World Example: Processing Student Scores

scores = [88, 92, 75, 91, 85]

avg = sum(scores) / len(scores)

print("Average Score:", avg)

📝 Practice Exercises

1. Create a list of 5 fruits and print the second and last elements.
2. Write a program to find the largest number in a list.
3. Remove all even numbers from a list using a loop.
4. Sort a list of names alphabetically.
5. Using list comprehension, create a list of squares from 1 to 10.

✅ Practice Answers

Answer 1:

fruits = ["apple", "banana", "mango", "orange", "grapes"]
print(fruits[1], fruits[-1])

Answer 2:

numbers = [10, 20, 55, 2, 90]
print(max(numbers))

Answer 3:

nums = [1,2,3,4,5,6,7,8,9]
result = []

for n in nums:
    if n % 2 != 0:
        result.append(n)

print(result)

Answer 4:

names = ["siri", "ram", "anil", "john"]
names.sort()
print(names)

Answer 5:

squares = [x*x for x in range(1, 11)]
print(squares)