🎒 Python Lists: Your Digital Backpack

Imagine you have a magical backpack. You can put anything inside—toys, snacks, books, even your pet rock! Python lists work exactly the same way. They’re containers that hold stuff for you.

🎯 What You’ll Learn

• How to create your own list (backpack)
• How to grab specific items from it
• How to change what’s inside
• How to check if something is there
• How to count how many things you have

📦 List Creation: Making Your Backpack

Creating a list is like getting a new backpack and deciding what to put inside.

The Magic Formula

my_list = [item1, item2, item3]

Square brackets [ ] are like the zipper of your backpack. Everything inside belongs together!

Examples

A backpack with fruits:

fruits = ["apple", "banana", "cherry"]

A backpack with numbers:

scores = [100, 85, 92, 78]

An empty backpack (ready to fill later):

empty_bag = []

A mixed backpack (Python doesn’t care!):

mixed = ["hello", 42, True, 3.14]

🧠 Remember This

• Use square brackets [ ]
• Separate items with commas ,
• Items can be anything—numbers, words, even other lists!

🔢 List Indexing: Finding Your Stuff

Your backpack has pockets numbered starting from 0 (not 1!). To grab something, just say which pocket number.

graph TD
    A["fruits = ['apple', 'banana', 'cherry']"] --> B["Index 0: apple"]
    A --> C["Index 1: banana"]
    A --> D["Index 2: cherry"]

Getting One Item

fruits = ["apple", "banana", "cherry"]
print(fruits[0])  # apple
print(fruits[1])  # banana
print(fruits[2])  # cherry

Negative Numbers: Counting Backwards!

What if you want the last item but don’t know how many things are in the bag?

fruits = ["apple", "banana", "cherry"]
print(fruits[-1])  # cherry (last)
print(fruits[-2])  # banana (second last)

-1 means “start from the end.”

✂️ List Slicing: Grabbing Multiple Items

Slicing is like saying, “Give me items from pocket A to pocket B.”

The Slice Formula

my_list[start:stop]

• start = where to begin (included)
• stop = where to end (NOT included)

Examples

letters = ["a", "b", "c", "d", "e"]

print(letters[1:4])   # ['b', 'c', 'd']
print(letters[:3])    # ['a', 'b', 'c']
print(letters[2:])    # ['c', 'd', 'e']
print(letters[:])     # ['a', 'b', 'c', 'd', 'e']

🎯 Quick Rules

🔄 List Mutability: Changing What’s Inside

Here’s something special about lists: you can change them!

This is called being mutable (changeable).

Swap One Item

colors = ["red", "green", "blue"]
colors[1] = "yellow"
print(colors)  # ['red', 'yellow', 'blue']

You just replaced “green” with “yellow”!

Change Multiple Items

numbers = [1, 2, 3, 4, 5]
numbers[1:4] = [20, 30, 40]
print(numbers)  # [1, 20, 30, 40, 5]

💡 Why This Matters

• Strings are immutable (can’t change individual letters)
• Lists are mutable (can change anything!)

# This works:
my_list = [1, 2, 3]
my_list[0] = 99

# This does NOT work:
my_string = "hello"
my_string[0] = "H"  # ERROR!

🔍 Checking Element Existence: Is It There?

Before reaching into your backpack, you might want to check if something is actually inside. Use the magic word: in

The Simple Check

fruits = ["apple", "banana", "cherry"]

if "banana" in fruits:
    print("Yes! We have banana!")
# Output: Yes! We have banana!

Checking What’s NOT There

fruits = ["apple", "banana", "cherry"]

if "mango" not in fruits:
    print("Sorry, no mango today")
# Output: Sorry, no mango today

🎮 Real Example

allowed_users = ["alice", "bob", "charlie"]
username = "bob"

if username in allowed_users:
    print("Welcome!")
else:
    print("Access denied!")
# Output: Welcome!

📏 List Length: Counting Your Items

How many things are in your backpack? Use len() to find out!

Basic Counting

fruits = ["apple", "banana", "cherry"]
print(len(fruits))  # 3

Why This Is Useful

Looping through all items:

fruits = ["apple", "banana", "cherry"]
for i in range(len(fruits)):
    print(f"Item {i}: {fruits[i]}")

Finding the last index:

fruits = ["apple", "banana", "cherry"]
last_index = len(fruits) - 1
print(fruits[last_index])  # cherry

Checking if list is empty:

my_list = []
if len(my_list) == 0:
    print("The backpack is empty!")

🎯 Putting It All Together

Let’s use everything we learned in one story!

# Create a shopping list
shopping = ["milk", "eggs", "bread"]

# Check how many items
print(f"Items to buy: {len(shopping)}")

# Add something? First check if it exists
if "butter" not in shopping:
    shopping.append("butter")

# Get the first and last items
first = shopping[0]
last = shopping[-1]

# Change the second item
shopping[1] = "cheese"

# Get items 1-3
middle_items = shopping[1:3]

print(shopping)
# ['milk', 'cheese', 'bread', 'butter']

🌟 Key Takeaways

🚀 You Did It!

You now understand Python lists—your digital backpack for storing anything you need. Go build something amazing!