📋 Lesson 7: Lists — Python's Super Memory

📦 What Are Lists?

So far, each variable you've created holds one value — one number, one string, one boolean. But what if you need to store a whole collection of things? Five test scores, a shopping list, a roster of player names?

A list is an ordered collection of items stored in a single variable. Each item has a numbered position (called an index) starting from 0.

🛠️ Creating Lists

Lists are created with square brackets []. They can hold any data type — and even mix types together:

# Empty list
shopping_cart = []

# List with items
fruits = ["apple", "banana", "cherry"]
numbers = [1, 2, 3, 4, 5]
mixed = ["hello", 42, 3.14, True]  # Lists can mix types!

# Lists can contain other lists!
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

🔍 Accessing List Items

Each item in a list has an address called an index. Python uses two numbering systems:

colors = ["red", "green", "blue", "yellow", "purple"]

# Positive indexing (from start)
print(colors[0])    # "red" (first item)
print(colors[2])    # "blue" (third item)

# Negative indexing (from end)
print(colors[-1])   # "purple" (last item)
print(colors[-2])   # "yellow" (second to last)

✂️ Slicing — Grabbing Multiple Items

Slicing lets you extract a portion of a list using the syntax [start:stop]. The stop index is not included.

colors = ["red", "green", "blue", "yellow", "purple"]

# Basic slicing
print(colors[1:4])  # ["green", "blue", "yellow"]
print(colors[:3])   # ["red", "green", "blue"] (first 3)
print(colors[2:])   # ["blue", "yellow", "purple"] (from index 2 on)

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

# Every 2nd item
print(numbers[::2])      # [0, 2, 4, 6, 8]

# Every 3rd item starting from index 1
print(numbers[1::3])     # [1, 4, 7]

# Reverse the list
print(numbers[::-1])     # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

# Reverse every 2nd item
print(numbers[::-2])     # [9, 7, 5, 3, 1]

original = [1, 2, 3]
copy1 = original[:]      # Full slice creates a copy ✅
copy2 = original.copy()  # Using copy method ✅
copy3 = list(original)   # Using list constructor ✅

# This does NOT copy — just creates another reference!
wrong_copy = original    # ❌
wrong_copy.append(4)
print(original)          # [1, 2, 3, 4] — original changed too!

🔧 Modifying Lists

Lists are mutable — you can change them after creation. Python gives you a toolbox of built-in methods:

List Methods in Action

# Starting with a todo list
todo = ["wake up", "coffee", "code"]
print(f"Original: {todo}")

# Adding items
todo.append("lunch")                    # Add to end
print(f"After append: {todo}")

todo.insert(2, "check email")           # Insert at position 2
print(f"After insert: {todo}")

# Removing items
todo.remove("coffee")                   # Remove specific item
print(f"After remove: {todo}")

completed = todo.pop(0)                 # Remove and return first item
print(f"Completed: {completed}")
print(f"After pop: {todo}")

# Other useful methods
todo.reverse()                          # Reverse the order
print(f"Reversed: {todo}")

print(f"'code' is at index: {todo.index('code')}")
print(f"Number of 'code' items: {todo.count('code')}")

Original: ['wake up', 'coffee', 'code']
After append: ['wake up', 'coffee', 'code', 'lunch']
After insert: ['wake up', 'coffee', 'check email', 'code', 'lunch']
After remove: ['wake up', 'check email', 'code', 'lunch']
Completed: wake up
After pop: ['check email', 'code', 'lunch']
Reversed: ['lunch', 'code', 'check email']
'code' is at index: 1
Number of 'code' items: 1

⏱️ Performance at a Glance

🔄 Lists and Loops — Perfect Partners

Lists and loops work together beautifully. You can process every item in a list automatically:

# Processing each item
scores = [85, 92, 78, 95, 88]

# Calculate average
total = 0
for score in scores:
    total += score
average = total / len(scores)
print(f"Average score: {average:.1f}")

# Find highest score
highest = scores[0]
for score in scores:
    if score > highest:
        highest = score
print(f"Highest score: {highest}")

Average score: 87.6
Highest score: 95

Looping with Index — enumerate()

# The long way — range(len(...))
print("Score Report:")
for i in range(len(scores)):
    print(f"  Test {i+1}: {scores[i]} points")

# The Pythonic way — enumerate!
print("\nUsing enumerate:")
for i, score in enumerate(scores):
    print(f"  Test {i+1}: {score} points")

✨ List Comprehensions — Python Magic

List comprehensions are a concise way to create new lists. They pack a loop (and optional condition) into a single line:

# Traditional way
squares = []
for i in range(1, 6):
    squares.append(i ** 2)
print(squares)  # [1, 4, 9, 16, 25]

# List comprehension — same result, one line!
squares = [i ** 2 for i in range(1, 6)]
print(squares)  # [1, 4, 9, 16, 25]

# With conditions — filter as you build
evens = [x for x in range(20) if x % 2 == 0]
print(evens)  # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

# Transform strings
fruits = ["apple", "banana", "cherry"]
uppercase = [fruit.upper() for fruit in fruits]
print(uppercase)  # ['APPLE', 'BANANA', 'CHERRY']

# Practical example: temperature conversion
temperatures_f = [32, 68, 75, 82, 90]
temperatures_c = [(f - 32) * 5/9 for f in temperatures_f]
print(f"Celsius: {[f'{c:.1f}' for c in temperatures_c]}")

Celsius: ['0.0', '20.0', '23.9', '27.8', '32.2']

🔀 Sorting — sort() vs sorted()

Python gives you two ways to sort. They look similar but behave differently:

# .sort() — modifies the list IN PLACE, returns None
numbers = [3, 1, 4, 1, 5, 9, 2, 6]
numbers.sort()
print(numbers)  # [1, 1, 2, 3, 4, 5, 6, 9] — original changed

# sorted() — returns a NEW sorted list, original untouched
numbers = [3, 1, 4, 1, 5, 9, 2, 6]
sorted_nums = sorted(numbers)
print(numbers)      # [3, 1, 4, 1, 5, 9, 2, 6] — original unchanged
print(sorted_nums)  # [1, 1, 2, 3, 4, 5, 6, 9] — new list

# Both support reverse and key parameters
words = ["banana", "pie", "Washington", "book"]
words.sort(key=len, reverse=True)
print(words)  # ['Washington', 'banana', 'book', 'pie']

# sorted() works on any iterable
print(sorted("python"))          # ['h', 'n', 'o', 'p', 't', 'y']
print(sorted({3, 1, 4, 1, 5}))  # [1, 3, 4, 5]

# Custom sorting with lambda
students = [("Alice", 85), ("Bob", 75), ("Carol", 95)]
students.sort(key=lambda x: x[1])
print(students)  # [('Bob', 75), ('Alice', 85), ('Carol', 95)]

📝 Lists vs Strings

Strings behave a lot like lists of characters — but there's one critical difference:

# Similarities — indexing, slicing, looping
text = "Python"
print(text[0])     # 'P'
print(text[-1])    # 'n'
print(text[1:4])   # 'yth'

# The big difference: mutability
my_list = ['P', 'y', 't', 'h', 'o', 'n']
my_list[0] = 'J'   # Works! Lists are mutable ✅
print(my_list)      # ['J', 'y', 't', 'h', 'o', 'n']

# text[0] = 'J'     # TypeError! Strings are immutable ❌

Converting Between Strings and Lists

# Split string → list
sentence = "Python is awesome"
words = sentence.split()
print(words)  # ['Python', 'is', 'awesome']

# Join list → string
joined = " ".join(words)
print(joined)  # "Python is awesome"

# Characters to list and back
chars = list("Hello")   # ['H', 'e', 'l', 'l', 'o']
word = "".join(chars)    # "Hello"

🧩 Common Patterns and Mistakes

Useful Patterns

# Pattern 1: Filtering
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
odds = [num for num in numbers if num % 2 != 0]
print(f"Odd numbers: {odds}")

# Pattern 2: Transformation
words = ["hello", "world", "python"]
lengths = [len(word) for word in words]
print(f"Word lengths: {lengths}")

# Pattern 3: Finding items
inventory = ["hammer", "nails", "screwdriver", "wrench"]
item = "screwdriver"
if item in inventory:
    print(f"Found {item} at position {inventory.index(item)}")

# Pattern 4: Removing duplicates
numbers = [1, 2, 2, 3, 3, 3, 4, 5, 5]
unique = list(set(numbers))
print(f"Unique: {unique}")

# Wrong
fruits = ["apple", "banana"]
# print(fruits[2])  # IndexError! Only indices 0 and 1

# Right — check length first
if len(fruits) > 2:
    print(fruits[2])
else:
    print("Not enough items")

# WRONG — modifying the list you're looping through
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)  # Dangerous! Skips items

# RIGHT — use a list comprehension instead
numbers = [1, 2, 3, 4, 5]
numbers = [num for num in numbers if num % 2 != 0]
print(numbers)  # [1, 3, 5]

🤔 When to Use Lists vs Other Structures

🛒 Project: Shopping List Manager

Let's put it all together! This project uses lists, loops, input, and if-statements — everything you've learned so far:

# Shopping List Manager
shopping_list = []

print("🛒 Shopping List Manager 🛒")
print("-" * 30)

while True:
    print("\nCurrent list:", shopping_list if shopping_list else "[Empty]")
    print("\nOptions:")
    print("1. Add item")
    print("2. Remove item")
    print("3. View list")
    print("4. Clear list")
    print("5. Exit")

    choice = input("\nEnter choice (1-5): ")

    if choice == "1":
        item = input("Enter item to add: ")
        shopping_list.append(item)
        print(f"Added '{item}' to list!")

    elif choice == "2":
        if shopping_list:
            print("Current items:")
            for i, item in enumerate(shopping_list):
                print(f"  {i+1}. {item}")
            try:
                index = int(input("Enter number to remove: ")) - 1
                removed = shopping_list.pop(index)
                print(f"Removed '{removed}'")
            except:
                print("Invalid selection!")
        else:
            print("List is empty!")

    elif choice == "3":
        if shopping_list:
            print("\n📋 Shopping List:")
            for i, item in enumerate(shopping_list, 1):
                print(f"  {i}. {item}")
        else:
            print("List is empty!")

    elif choice == "4":
        shopping_list.clear()
        print("List cleared!")

    elif choice == "5":
        print("Happy shopping! 👋")
        break

    else:
        print("Invalid choice!")

🏋️ Practice Exercises

scores = []

print("📝 Grade Calculator")
print("Enter scores one at a time. Type 'done' when finished.\n")

while True:
    entry = input("Enter a score: ")
    if entry.lower() == "done":
        break
    try:
        scores.append(float(entry))
    except ValueError:
        print("Please enter a number or 'done'.")

if scores:
    average = sum(scores) / len(scores)
    print(f"\n--- Results ({len(scores)} scores) ---")
    print(f"Average:  {average:.1f}")
    print(f"Highest:  {max(scores)}")
    print(f"Lowest:   {min(scores)}")

    # Drop lowest and recalculate
    adjusted = scores.copy()
    adjusted.remove(min(adjusted))
    new_avg = sum(adjusted) / len(adjusted)
    print(f"\nAfter dropping lowest ({min(scores)}):")
    print(f"New average: {new_avg:.1f}")
else:
    print("No scores entered.")

import random

playlist = []
current = 0

print("🎵 Playlist Manager 🎵")

while True:
    print(f"\nPlaylist: {playlist if playlist else '[Empty]'}")
    if playlist:
        print(f"Now playing: {playlist[current]}")
    print("\n1. Add song  2. Remove song  3. Shuffle")
    print("4. Next  5. Previous  6. Quit")

    choice = input("Choice: ")

    if choice == "1":
        song = input("Song name: ")
        playlist.append(song)
        print(f"Added '{song}'")

    elif choice == "2" and playlist:
        for i, s in enumerate(playlist, 1):
            print(f"  {i}. {s}")
        try:
            idx = int(input("Number to remove: ")) - 1
            removed = playlist.pop(idx)
            current = min(current, len(playlist) - 1)
            print(f"Removed '{removed}'")
        except:
            print("Invalid!")

    elif choice == "3" and playlist:
        random.shuffle(playlist)
        current = 0
        print("Playlist shuffled!")

    elif choice == "4" and playlist:
        current = (current + 1) % len(playlist)

    elif choice == "5" and playlist:
        current = (current - 1) % len(playlist)

    elif choice == "6":
        print("Rock on! 🎸")
        break

import random

words = ["python", "coding", "variable", "function", "loops"]
secret = random.choice(words)
revealed = ["_"] * len(secret)
guessed = []
attempts = 6

print("🔮 Word Guessing Game!")
print(f"The word has {len(secret)} letters.")
print(f"Hint: it starts with '{secret[0]}'")
revealed[0] = secret[0]

while attempts > 0 and "_" in revealed:
    print(f"\nWord: {' '.join(revealed)}")
    print(f"Guessed: {', '.join(guessed) if guessed else 'none'}")
    print(f"Attempts left: {attempts}")

    guess = input("Guess a letter: ").lower()

    if len(guess) != 1 or not guess.isalpha():
        print("Enter a single letter.")
        continue
    if guess in guessed:
        print("Already guessed!")
        continue

    guessed.append(guess)

    if guess in secret:
        for i, letter in enumerate(secret):
            if letter == guess:
                revealed[i] = guess
        print(f"✅ '{guess}' is in the word!")
    else:
        attempts -= 1
        print(f"❌ '{guess}' is not in the word.")

if "_" not in revealed:
    print(f"\n🎉 You got it! The word was '{secret}'!")
else:
    print(f"\n😢 Out of attempts. The word was '{secret}'.")

📚 Summary

📌 Real-World Applications

Lists are everywhere in real Python code: storing survey responses and measurements (data analysis), tracking inventory items and high scores (games), managing user lists and product catalogs (web development), queuing tasks and file lists (automation), and holding training data and predictions (machine learning).

🚀 What's Next?

Now that you can store multiple items, we'll explore dictionaries — Python's way of storing data with labels. Think of them as lists where items have names instead of numbers!