Object-Oriented Programming
Home Lesson 1 — Classes, Attributes and the Constructor

Start here

How this lesson works

Paper 4 is sat at a computer. You cannot learn it by reading about it, so this page teaches, then makes you type.

1. Learn on this page. Predict what code will do, run it here in the browser, then pick it apart. Nothing to install.
2. Build it in Python on your own machine. The page's runner is for experimenting. Your assessed code is written in IDLE, Thonny or VS Code, exactly as it will be in the exam.
3. Evidence it. Paste your code as text and your screenshots into the Evidence Document, and submit it on Teams. That document is a copy of the one Cambridge makes you fill in on exam day, so you are practising the exam admin as well as the code.

Retrieval

Do Now — three minutes, no notes

Answer these before you read anything else. Write them in your book, then reveal the answers and mark them.

Where this is going

Learning objectives

Vocabulary

Key terms — learn these wordings

These are written the way a mark scheme writes them. A looser paraphrase of the right idea scores nothing, so learn the wording, not the gist.

PRIMM · Step 1

Predict

Read this program carefully. Do not run it yet. Write down exactly what you think it will output, character for character.

class Book:
    def __init__(self, Title, Author):
        self.__Title = Title        # STRING
        self.__Author = Author      # STRING
        self.__OnLoan = False       # BOOLEAN

    def Display(self):
        print("Title : " + self.__Title)
        print("Author: " + self.__Author)
        print("On loan: " + str(self.__OnLoan))

MyBook = Book("Dune", "Frank Herbert")
MyBook.Display()

PRIMM · Step 2

Run

Now run it. Press Run and compare the real output with your prediction. Python runs inside this page — nothing is sent anywhere, and you can break it as much as you like.

Press Run to execute.

Your code and your prediction are saved in this browser, so closing the tab does not lose them. If a program never finishes, press Stop.

Title : Dune
Author: Frank Herbert
On loan: False

print(MyBook.__Title)

PRIMM · Step 3

Investigate — the four parts of a class

1. The class header

class Book: — the name is a singular noun with a capital letter. Book, not Books, and not BookClass. The class describes one of the thing; you make as many objects from it as you like.

2. Private attributes, with their data types visible

Python has no PRIVATE keyword and no type declarations, so you have to show the examiner what a VB.NET or Java candidate gets for free:

Write both, every time, on every attribute. It costs you four seconds and it is a mark.

3. The constructor

In Python the constructor is always called __init__ — two underscores, init, two underscores. Its first parameter is always self.

4. A method

Display() is a method: a subroutine that belongs to the class. Its first parameter is self, and it is called on an object with a dot — MyBook.Display().

PRIMM · Step 4

Modify

Go back to the runner above and change the Book class so that it matches this class design. This is exactly the form Cambridge gives you a class in, so read it the way you will read it in the exam.

Then: update Display() to output all five attributes with labels, and create a second object of your own choosing so that two different books are displayed.

class Book:
    def __init__(self, Title, Author, Year):
        self.__Title = Title             # STRING
        self.__Author = Author           # STRING
        self.__Year = Year               # INTEGER
        self.__OnLoan = False            # BOOLEAN
        self.__TimesBorrowed = 0         # INTEGER

    def Display(self):
        print("Title          : " + self.__Title)
        print("Author         : " + self.__Author)
        print("Year           : " + str(self.__Year))
        print("On loan        : " + str(self.__OnLoan))
        print("Times borrowed : " + str(self.__TimesBorrowed))

FirstBook = Book("Dune", "Frank Herbert", 1965)
SecondBook = Book("Emma", "Jane Austen", 1815)
FirstBook.Display()
SecondBook.Display()

The marks

Exam technique — the class design table is the instruction

Almost everything that goes wrong on an OOP question goes wrong before any code is written, because the candidate implemented the class they remembered rather than the class they were given. Cambridge does this deliberately: the designs change slightly every year specifically to test whether you read them.

Predict, then reveal. A candidate is given the Book design above and writes this. Decide what an examiner would say before you open the answer.

class Book:
    def __init__(self, Title, Author, Year, OnLoan, TimesBorrowed):
        self.Title = Title
        self.Author = Author
        self.Year = Year
        self.OnLoan = OnLoan
        self.TimesBorrowed = TimesBorrowed

Assessed work

Over to you — write it in Python, evidence it in the document

Everyone starts at Independent. Drop to Scaffolded if you stall. Go to Exam Conditions when you have finished — and by the mock, that is where everyone needs to be.

class Member:
    def __init__(self, ______, ______, ______):
        self.__MemberID = ______      # STRING
        # ... Name
        # ... Age
        # ... Credits  (fixed value — not a parameter)
        # ... Active   (fixed value — not a parameter)

    def DisplayDetails(self):
        print("Member ID : " + self.__MemberID)
        # ... one print for each remaining attribute
        # ... remember str() around the ones that are not strings

# ... create the object here, with a sensible variable name
# ... call DisplayDetails on it

Self-marking

Mark scheme — open it only after you have attempted the task

Mark your own work honestly against this, then record your total in the Evidence Document. Marking your own work against the real wording is how you learn what "one mark per point, max 8" actually means.

Hand-in

What to submit on Teams

One file: the Evidence Document for this lesson, saved as .docx and named Surname Initial — OOP Home Lesson 1.docx.

Your next taught lesson opens by checking these submissions, so an empty one is visible immediately. If you got stuck, submit what you have with a note saying where — that is far more useful to me than nothing.

Coming next

Home Lesson 2 — Get and set methods. Getting a value out of a private attribute and putting a new one in. It sounds trivial. A get method written with a parameter is the single most repeated OOP error in the examiner reports — all ten recent series — so it is worth a lesson of its own.