Objective

In this unit, we will learn about lists in Python. Lists are a type of data structure that allow you to store multiple items in a single variable. By the end of this unit, you will understand how to create, access, and modify lists, and you will know about some of the most commonly used list methods.

Understanding Lists

A list is a collection of items that are ordered and changeable. Lists allow duplicate items. In Python, lists are written with square brackets [], and items are separated by commas.

Here's an example of a list:

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

This will output: ['apple', 'banana', 'cherry']

Creating and Accessing Lists

You can create a list by enclosing a comma-separated sequence of items in square brackets []. The items can be of different types (e.g., integers, strings, booleans, etc.).

my_list = [1, "Hello", True]

In this example, my_list is a list that contains an integer (1), a string ("Hello"), and a boolean (True).

You can access the items in a list by referring to their index number. Indices in Python start at 0, so the first item has an index of 0, the second item has an index of 1, and so on.

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

Modifying Lists

You can change the value of a specific item in a list by referring to its index number.

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

In this example, we changed the second item in the fruits list from "banana" to "orange".

List Methods

Python provides several methods that you can use to manipulate lists. Here are a few commonly used ones:

  • append(item): Adds an item to the end of the list.
  • remove(item): Removes the first occurrence of the item from the list.
  • sort(): Sorts the items in the list in ascending order.
  • reverse(): Reverses the order of the items in the list.

Here's an example of how to use these methods:

numbers = [3, 1, 4, 1, 5, 9]

numbers.append(2)
print(numbers)  # Prints: [3, 1, 4, 1, 5, 9, 2]

numbers.remove(1)
print(numbers)  # Prints: [3, 4, 1, 5, 9, 2]

numbers.sort()
print(numbers)  # Prints: [1, 2, 3, 4, 5, 9]

numbers.reverse()
print(numbers)  # Prints: [9, 5, 4, 3, 2, 1]

Project: Use a List to Store Multiple Shapes and Have Your Turtle Draw Each of Them in Order

In this project, we will be utilizing the power of lists along with the Turtle graphics library to draw a sequence of shapes. The challenge here is to apply the concept of lists creatively.

import turtle

# Create a new turtle screen, set its size and background color
screen = turtle.Screen()
screen.setup(width=800, height=600)  # Set the width and height of the window
screen.bgcolor("white")

# Initialize a turtle object
t = turtle.Turtle()

# Function to draw a shape with a given number of sides
def draw_shape(turtle, side_length, num_sides):
    angle = 360 / num_sides
    for _ in range(num_sides):
        turtle.forward(side_length)
        turtle.right(angle)

# List of shapes to draw (each shape is represented by the number of its sides)
shapes = [3, 4, 5, 6, 8]

# Move the turtle to the left edge of the screen
t.penup()
t.goto(-350, 0)  # x=-350, y=0
t.pendown()

# Draw each shape in the list
for shape in shapes:
    draw_shape(t, 50, shape)
    t.penup()
    t.forward(140)  # Move the turtle to the right
    t.pendown()

# Hide the turtle and wait until the window is closed
t.hideturtle()
turtle.done()

In the project code above, we first define a function draw_shape that draws a shape with a given number of sides. We then create a list shapes that contains the number of sides for each shape we want to draw. We use a for loop to iterate over this list and draw each shape by calling the draw_shape function.

In the next unit, we'll look at list comprehension and explore how we can generate lists in a more concise way.