Python Programming · Week 5

Lists

Step 1 of 6

In Python, a list is a versatile and powerful data structure that allows you to store multiple items in a single variable. Think of a list as a container that can hold various types of data, much like a shopping list can contain different items.

Lists are:

Ordered: Items have a defined order, and that order will not change.
Mutable: You can change, add, and remove items in a list after it is created.
Allow duplicates: Since lists are indexed, they can have items with the same value.

Example of a List:

my_list = [1, "apple", 3.14, True]

In this example, we've created a list called 'my_list' that contains four different types of data:

An integer (1)
A string ("apple")
A float (3.14)
A boolean (True)

This demonstrates how flexible lists can be in Python.

Real-world Analogy

Imagine a backpack. You can put different items in it: books, pens, a water bottle, and snacks. Each item can be different, just like in a Python list. You can add or remove items from your backpack as needed, similar to how you can modify a list.

Try it Yourself

Now, let's create three different lists and print them. Follow these steps:

Create a list of fruits
Create a list of numbers
Create a mixed list with different types of data
Print each list

Hint: To create a list, use square brackets [ ] and separate items with commas. To print a list, you can simply use the print() function with the list name as an argument.

Starter code

# Step 1: Create a list of fruits
fruits = ["apple", "banana", "orange"]

# Step 2: Create a list of numbers
numbers = [1, 2, 3, 4, 5]

# Step 3: Create a mixed list
mixed = [True, "Python", 3.14, [1, 2, 3]]

# Step 4: Print the lists
print("Fruits:", fruits)
print("Numbers:", numbers)
print("Mixed:", mixed)

Write your code here

Declaring a list in Python is straightforward. You use square brackets [ ] to enclose the list items, separating each item with a comma.

Syntax for Declaring a List:

list_name = [item1, item2, item3, ...]

Here's an example of declaring a list of groceries:

groceries = ["Milk", "Bread", "Eggs", "Cheese"]

In this example:

'groceries' is the name of our list
The items "Milk", "Bread", "Eggs", and "Cheese" are the elements of the list
Each element is a string, enclosed in quotes
The elements are separated by commas

Lists Can Contain Different Data Types

Remember, lists in Python can contain items of different data types. For example:

mixed_list = ["Hello", 42, 3.14, True]

This list contains a string, an integer, a float, and a boolean.

Try it Yourself

Now, let's create a list of your favorite books and perform some operations on it. Follow these steps:

Create a list called 'favorite_books' with at least four book titles
Print the list of favorite books
Print the number of books in the list using the len() function
Add a new book to the list using the append() method
Print the updated list and the new number of books

Hint: To add an item to a list, you can use the append() method. The syntax is: list_name.append(new_item)

Starter code

# Step 1: Create a list of your favorite books
favorite_books = ["To Kill a Mockingbird", "1984", "Pride and Prejudice", "The Great Gatsby"]

# Step 2: Print the list
print("My favorite books:", favorite_books)

# Step 3: Print the number of books
print("Number of favorite books:", len(favorite_books))

# Step 4: Add a new book to the list
favorite_books.append("The Catcher in the Rye")

# Step 5: Print the updated list and new number of books
print("Updated list of favorite books:", favorite_books)
print("New number of favorite books:", len(favorite_books))

Write your code here

In Python lists, each item has a specific position, called an index. Understanding indexes is crucial for accessing and manipulating list items.

Key Points About Indexes:

Indexing starts at 0, not 1
The last item in the list has an index of -1
You can access items using their index: list_name[index]

Example of Indexing:

fruits = ["apple", "banana", "orange", "grape"]
# Indexes:    0        1         2        3
            

In this example:

"apple" is at index 0
"banana" is at index 1
"orange" is at index 2
"grape" is at index 3

You can also use negative indexing:

"grape" is at index -1 (last item)
"orange" is at index -2 (second to last item)
And so on...

Try it Yourself

Let's practice working with list indexes. Follow these steps:

Create a list of colors
Print each element with its positive index
Print the first and last elements using positive and negative indexing
Try to access an index that doesn't exist and see what happens

Hint: To print each element with its index, you can use a for loop with the enumerate() function. For example:

for index, item in enumerate(list_name):
    print(f"Index {index}: {item}")

Starter code

# Step 1: Create a list of colors
colors = ["red", "blue", "green", "yellow", "purple"]

# Step 2: Print each element with its index
for index, color in enumerate(colors):
    print(f"Index {index}: {color}")

# Step 3: Print the first and last elements
print("\nFirst color (positive index):", colors[0])
print("Last color (positive index):", colors[4])
print("First color (negative index):", colors[-5])
print("Last color (negative index):", colors[-1])

# Step 4: Try to access an index that doesn't exist
try:
    print("\nTrying to access index 5:", colors[5])
except IndexError as e:
    print("\nError occurred:", str(e))

Write your code here

Python provides a variety of built-in functions and methods to manipulate lists. Understanding these functions is essential for working effectively with lists.

Common List Functions and Methods:

len(list): Returns the number of items in the list
list.append(item): Adds an item to the end of the list
list.insert(index, item): Inserts an item at a specified position
list.remove(item): Removes the first occurrence of the specified item
list.pop(index): Removes and returns the item at the specified position
list.index(item): Returns the index of the first occurrence of the specified item
list.count(item): Returns the number of times the specified item appears in the list
list.sort(): Sorts the list in ascending order
list.reverse(): Reverses the order of the list

Try it Yourself

Let's practice using these list functions. Follow these steps:

Create a list of numbers
Print the original list
Add a new number to the end of the list
Insert a number at the beginning of the list
Remove a specific number from the list
Sort the list in ascending order
Reverse the list
Print the final list

Hint: Remember to print the list after each operation to see how it changes. You can use the print() function with a descriptive message for each step.

Starter code

# Step 1: Create a list of numbers
numbers = [5, 2, 8, 1, 9, 3, 7]

# Step 2: Print the original list
print("Original list:", numbers)

# Step 3: Add a new number to the end of the list
numbers.append(6)
print("After appending 6:", numbers)

# Step 4: Insert a number at the beginning of the list
numbers.insert(0, 0)
print("After inserting 0 at the beginning:", numbers)

# Step 5: Remove a specific number from the list
numbers.remove(9)
print("After removing 9:", numbers)

# Step 6: Sort the list in ascending order
numbers.sort()
print("After sorting:", numbers)

# Step 7: Reverse the list
numbers.reverse()
print("After reversing:", numbers)

# Step 8: Print the final list
print("Final list:", numbers)

# Bonus: Let's use some other list functions
print("\nBonus operations:")
print("Number of items in the list:", len(numbers))
print("Index of 7 in the list:", numbers.index(7))
print("Count of 3 in the list:", numbers.count(3))

Write your code here

The "in" operator in Python is a powerful tool for checking if an item exists in a list. It returns True if the item is found in the list, and False otherwise.

Syntax:

item in list_name

This expression evaluates to True if 'item' is found in 'list_name', and False otherwise.

Example:

fruits = ["apple", "banana", "orange"]
if "banana" in fruits:
    print("Yes, banana is in the fruits list")
else:
    print("No, banana is not in the fruits list")
            

The "in" operator is often used in conditional statements and loops to check for the presence of an item before performing an operation.

Try it Yourself

Let's practice using the "in" operator with lists. Follow these steps:

Create a list of fruits
Create a list of fruits to check
Use a loop to check if each fruit in the check list is in the fruits list
Add a new fruit to the list
Check if the new fruit is now in the list

Hint: You can use a for loop to iterate through the list of fruits to check. Inside the loop, use an if statement with the "in" operator to check if each fruit is in the main fruits list.

Starter code

# Step 1: Create a list of fruits
fruits = ["apple", "banana", "orange", "grape", "kiwi"]

# Step 2: Create a list of fruits to check
fruits_to_check = ["apple", "mango", "kiwi", "pear"]

# Step 3: Check if each fruit in the check list is in the fruits list
print("Checking for fruits:")
for fruit in fruits_to_check:
    if fruit in fruits:
        print(f"{fruit} is in the fruits list")
    else:
        print(f"{fruit} is not in the fruits list")

# Step 4: Add a new fruit to the list
new_fruit = "mango"
fruits.append(new_fruit)
print(f"\nAdded {new_fruit} to the list")

# Step 5: Check if the new fruit is now in the list
if new_fruit in fruits:
    print(f"{new_fruit} is now in the fruits list")
else:
    print(f"{new_fruit} is still not in the fruits list")

# Bonus: Print the updated list
print("\nUpdated fruits list:", fruits)

Write your code here

Now that we've learned about lists and their operations, let's put our knowledge into practice by creating a simple To-Do List Manager. This project will help reinforce the concepts we've covered and show how they can be applied in a real-world scenario.

Project Requirements:

Create a To-Do List Manager that can:

Add tasks to the list
Remove tasks from the list
Display all tasks
Mark tasks as completed

Implementation Steps:

Create two lists: one for active tasks and one for completed tasks
Implement a function to add tasks to the active list
Implement a function to remove tasks from the active list
Implement a function to display all tasks (both active and completed)
Implement a function to mark tasks as completed (move from active to completed list)
Test your functions by adding, removing, completing, and displaying tasks

Hint: Use the list methods we've learned (append, remove) to manipulate the task lists. When displaying tasks, you can use enumeration to show task numbers.

The template below provides a starting point for your To-Do List Manager. Fill in the functions and add any additional functionality you think would be useful!

Starter code

# To-Do List Manager

tasks = []
completed_tasks = []

def add_task(task):
    tasks.append(task)
    print(f"Task '{task}' added to the list.")

def remove_task(task):
    if task in tasks:
        tasks.remove(task)
        print(f"Task '{task}' removed from the list.")
    else:
        print(f"Task '{task}' not found in the list.")

def display_tasks():
    if tasks:
        print("Current tasks:")
        for index, task in enumerate(tasks, 1):
            print(f"{index}. {task}")
    else:
        print("No tasks in the list.")

    if completed_tasks:
        print("\nCompleted tasks:")
        for index, task in enumerate(completed_tasks, 1):
            print(f"{index}. {task}")

def mark_completed(task):
    if task in tasks:
        tasks.remove(task)
        completed_tasks.append(task)
        print(f"Task '{task}' marked as completed.")
    else:
        print(f"Task '{task}' not found in the list.")

# Test your functions
add_task("Buy groceries")
add_task("Finish homework")
add_task("Call mom")

print("\nInitial task list:")
display_tasks()

remove_task("Finish homework")
mark_completed("Buy groceries")

print("\nUpdated task list:")
display_tasks()

# Add more tasks and interact with the list
add_task("Go for a run")
add_task("Read a book")
mark_completed("Call mom")

print("\nFinal task list:")
display_tasks()

Write your code here

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