Python List

Python Lists: A Complete Introduction

Lists are one of the most versatile and commonly used data types in Python. They allow you to store and manipulate a collection of items in a single variable.

What Is a List?

A list is an ordered collection of items that can hold elements of any data type - strings, numbers, other lists, or even mixed types.

my_list = [1, 2, 3, "hello", [4, 5]]

• Lists are mutable, meaning you can change their contents after creation.
• They are defined using square brackets [].
• Each element is separated by a comma.

Creating a List

You can create an empty list or a list with elements:

empty_list = []
numbers = [10, 20, 30]
fruits = ["apple", "banana", "cherry"]

Accessing Elements in a List

You can access elements using indexing, where the first element has index 0:

fruits = ["apple", "banana", "cherry"]
print(fruits[0])  # Output: apple
print(fruits[2])  # Output: cherry

You can also use negative indexing to access elements from the end:

print(fruits[-1])  # Output: cherry
print(fruits[-2])  # Output: banana

Modifying a List

Lists are mutable, so you can change their content:

fruits = ["apple", "banana", "cherry"]
fruits[1] = "blueberry"
print(fruits)  # Output: ['apple', 'blueberry', 'cherry']

Common List Operations

Here are some useful operations and methods for working with lists:

Adding Elements

fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits)  # Output: ['apple', 'banana', 'cherry']

fruits.insert(1, "orange")
print(fruits)  # Output: ['apple', 'orange', 'banana', 'cherry']

Removing Elements

fruits = ["apple", "banana", "cherry"]
fruits.remove("banana")
print(fruits)  # Output: ['apple', 'cherry']

popped = fruits.pop()
print(popped)  # Output: cherry
print(fruits)  # Output: ['apple']

Slicing a List

You can extract a portion of a list using slicing:

numbers = [0, 1, 2, 3, 4, 5]
print(numbers[1:4])  # Output: [1, 2, 3]
print(numbers[:3])   # Output: [0, 1, 2]
print(numbers[3:])   # Output: [3, 4, 5]

Iterating Through a List

You can use a for loop to go through each item:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

Checking for Membership

Use in to check if an item is in the list:

fruits = ["apple", "banana", "cherry"]
print("banana" in fruits)  # Output: True
print("grape" in fruits)   # Output: False

List Comprehension

List comprehension is a concise way to create lists:

squares = [x**2 for x in range(5)]
print(squares)  # Output: [0, 1, 4, 9, 16]

This is equivalent to:

squares = []
for x in range(5):
    squares.append(x**2)

Sorting and Reversing

You can sort and reverse lists easily:

numbers = [3, 1, 4, 2]
numbers.sort()
print(numbers)  # Output: [1, 2, 3, 4]

numbers.reverse()
print(numbers)  # Output: [4, 3, 2, 1]

Note: sort() modifies the list in-place. Use sorted(numbers) to get a new sorted list.

Nested Lists

Lists can contain other lists:

matrix = [
    [1, 2],
    [3, 4],
    [5, 6]
]

print(matrix[1])     # Output: [3, 4]
print(matrix[1][0])  # Output: 3

Useful List Functions

• len(list) – number of elements
• max(list) – largest item (numbers or alphabetical)
• min(list) – smallest item
• sum(list) – sum of all elements (if numeric)

numbers = [10, 20, 30]
print(len(numbers))  # Output: 3
print(sum(numbers))  # Output: 60

Summary

• Lists are ordered, mutable collections.
• You can access, modify, slice, and iterate through them.
• Use methods like append(), insert(), remove(), and pop() for common tasks.
• List comprehension makes list creation concise and powerful.
• You can sort, reverse, and nest lists for more advanced structures.

Understanding lists is essential to mastering Python. They're foundational for almost every project or script you'll write.