Object Oriented Programming: Classes and Objects

1 Explain the difference between a class and an object, using an example. 3

One mark for each of the following, to a maximum of 3.

• A class is a template or blueprint defining attributes and methods
• An object is a specific instance created from that class
• Gives a valid example such as a Dog class with individual dogs as objects

Model answer. A class is a blueprint that defines what attributes and methods every object of that type will have, but it does not itself hold any particular data. An object is a specific instance created from that class, with its own values for each attribute. For example, a Dog class might define that every dog has a name and a breed and can bark. Creating rex = Dog("Rex", "Labrador") and bella = Dog("Bella", "Collie") produces two separate objects from that one class, sharing the same methods but each holding its own name and breed.

2 Write a class called Rectangle with a constructor taking width and height, and a method that returns the area. 5

One mark for each of the following, to a maximum of 5.

• Defines the class with a suitable name
• Defines __init__ with self, width and height
• Assigns the parameters to attributes using self
• Defines a method taking self
• Returns width multiplied by height

Model answer. class Rectangle: def __init__(self, width, height): self.width = width self.height = height def area(self): return self.width * self.height r = Rectangle(5, 3) print(r.area()) # 15 The constructor runs automatically when a Rectangle is created and stores the width and height as attributes of that particular object. The area method takes self as its first parameter so that it can access those attributes, and it returns the result rather than printing it, so the value can be used in further calculations.

3 Explain what is meant by encapsulation and why it is useful. 4

One mark for each of the following, to a maximum of 4.

• Data and the methods that operate on it are bundled together in a class
• Access to the data from outside is controlled
• The object can therefore validate any change to its data
• This prevents other parts of the program putting the object into an invalid state

Model answer. Encapsulation means bundling an object's data together with the methods that operate on that data inside a single class, and controlling how the data can be accessed from outside. Rather than allowing any part of the program to change an attribute directly, changes are made through methods or properties belonging to the class. This is useful because it means the object can validate every change before accepting it, so a bank account can refuse a withdrawal that would take the balance below zero, and a temperature can refuse a value below absolute zero. Because the rule lives inside the class, it applies automatically everywhere the class is used, and no other part of the program can accidentally or deliberately put the object into a state that should be impossible.

4 Explain the purpose of the self parameter in a Python class. 3

One mark for each of the following, to a maximum of 3.

• It refers to the particular object the method is being called on
• It allows the method to access that object's own attributes
• Different objects therefore produce different results from the same method

Model answer. The self parameter refers to the specific object on which a method has been called. When rex.bark() is executed, Python passes rex in as self, so inside the method self.name refers to rex's name rather than any other dog's. This is what allows every object created from the same class to share the same method code while behaving differently, because each one has its own set of attribute values. Without self, a method would have no way of knowing whose data it was supposed to be working with.

5 A class stores a student's mark. Describe how you would ensure the mark can never be set to a value outside 0 to 100. 4

One mark for each of the following, to a maximum of 4.

• Store the mark in a private attribute with a leading underscore
• Provide a setter method or property to change it
• Validate the value inside that method before assigning it
• Reject or raise an error for invalid values

Model answer. The mark would be stored in a private attribute such as self._mark, with the leading underscore signalling that it is internal and should not be accessed directly from outside the class. Access would then be provided through a property, with a getter that simply returns the value and a setter that checks the new value before accepting it. The setter would test whether the value is between 0 and 100 inclusive and assign it only if so, raising a ValueError otherwise. Because the check lives inside the class, every piece of code that ever sets a mark goes through it automatically, so it is impossible for any part of the program to leave a student object holding an invalid mark.