Introduction

In the previous lesson, you learned how to:

  • Create methods inside a class
  • Use self to access object data
  • Call methods on different objects

So far, our methods have only displayed information.
In this lesson, you’ll learn how methods can also change the data inside an object.

You will learn how to:

  • Define multiple methods inside a class
  • Use methods to update an object’s attributes
  • Understand how an object’s state can change over time

What Is State?

The state of an object refers to the data it currently holds.

For example:

  • A student’s name
  • A student’s year group
  • A student’s score

When a method changes one of these values, the state of the object changes.

Example: Updating an Object’s State 

class Student:
    def __init__(self, name, score):
        self.name = name
        self.score = score

    def add_points(self):
        self.score += 10

student = Student('Alex', 50)
student.add_points()

print(student.score)

Here:

  • The object starts with a score of 50
  • The method add_points changes the score
  • The object now stores a new value

Predict:

Look at the following code. What do you think will be printed?

class Student:
    def __init__(self, name, score):
        self.name = name
        self.score = score

    def add_points(self):
        self.score += 5

student = Student('Jamie', 40)
student.add_points()
student.add_points()

print(student.score)

Think about:

  • How many times is the method called?
  • Does the score reset or keep increasing?

Run:

Run the code below and check your prediction.

Investigate:

Now let’s investigate how multiple methods can affect the same object.

Try this:

  • Add a new method called remove_points
  • This method should subtract 5 from the score
  • Call both methods in different orders

Use the editor below to experiment.

Think about:

  • Does the order of method calls matter?
  • Why does the object remember its updated score?

Modify:

The following program is meant to update the student’s score, but it contains an error.

Fix the code so the score changes correctly.

The problem

  • The method does not update the object’s attribute
  • self is missing when changing the score
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