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Free A-Level Computer Science guide
Build on classes, objects and encapsulation by following what happens when different kinds of ticket respond to the same method call.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
Inheritance defines a specialised class using an existing class. The subclass can reuse attributes and methods and override a method when it needs a different implementation. An appropriate relationship means the specialised object can still fulfil the general class's role.
Polymorphism lets a caller use a shared operation on different kinds of object while each object supplies its appropriate behaviour. In the example, every ticket supports cost_pence(), so the checkout loop does not need separate pricing branches for each ticket type.
OCR H446 · 1.2.4(e), 2.2.1(f)
Useful foundations
By the end, you should be able to
A science exhibition sells standard and discounted student tickets. Each has a holder, base price and cost operation. Checkout expects that shared interface.
A shared field alone does not justify inheritance. Use it when the specialised object can fulfil the general role. Use composition when an object contains or uses another.
The example uses integer pence and truncates fractional-penny discounts through integer division. This is a stated pricing rule; a real system must define its own rounding policy.
class StudentTicket(Ticket) declares inheritance. Its constructor calls super().__init__(holder, base_pence) for common fields, then stores the discount percentage. This single-inheritance example reaches the superclass; more complex hierarchies are outside its scope.
Ticket.cost_pence returns base price. The subclass overrides that method with the same name and caller-facing arguments to apply a discount. Without an override, it would inherit the base implementation.
super().cost_pence() reuses the base method on the same object. It does not turn the object into a superclass object. Calling self.cost_pence() inside the override instead would call itself repeatedly.
Checkout calls one operation on every ticket. The appropriate implementation supplies the cost without an “if student” pricing branch. Another class meeting that contract can leave the loop unchanged.
Python can also use unrelated objects sharing suitable operations. Here inheritance makes reuse explicit. Polymorphism means the same call supports differing appropriate behaviours, rather than merely two class names.
A single underscore marks a Python non-public convention, without enforcing privacy. Methods and constructors protect invariants here: base price cannot be negative and discount must be 0…100.
ticket.cost_pence() uses Ticket’s method: 1000 pence.Checkout uses the shared operation; the object’s class determines its behaviour.
Worked example
| Line | Output |
|---|---|
| 1 | Ada 1000 |
| 2 | Ben 750 |
| 3 | Cy 400 |
| 4 | Total 2150 |
class Ticket:
def __init__(self, holder, base_pence):
if base_pence < 0:
raise ValueError("negative price")
self.holder = holder
self._base_pence = base_pence
def cost_pence(self):
return self._base_pence
class StudentTicket(Ticket):
def __init__(self, holder, base_pence, discount_percent):
super().__init__(holder, base_pence)
if not 0 <= discount_percent <= 100:
raise ValueError("discount outside 0..100")
self._discount_percent = discount_percent
def cost_pence(self):
base = super().cost_pence()
return base * (100 - self._discount_percent) // 100
tickets = [Ticket("Ada", 1000),
StudentTicket("Ben", 1000, 25),
StudentTicket("Cy", 800, 50)]
for ticket in tickets:
print(ticket.holder, ticket.cost_pence())
print("Total", sum(ticket.cost_pence() for ticket in tickets))Worked example
5 original questions total 19 marks. Attempt each before opening the independently written indicative marking guidance.
Identify the superclass and subclass in the example. Explain one reused feature and one specialised feature.
Indicative answer
A StudentTicket has a base price of 1200 pence and a 30 percent discount. Give its cost and explain the two cost_pence implementations involved.
Indicative answer
Explain where the runnable program demonstrates polymorphism and why the checkout can work with another suitable ticket subclass.
Indicative answer
A Booking stores several tickets and a confirmation code. Decide whether it should inherit from Ticket or contain ticket objects, giving two reasons.
Indicative answer
Give expected outcomes for a 0 percent discount, a 100 percent discount, a 101 percent discount and a negative base price. All other values are valid integers.
Indicative answer
This guide addresses OCR H446 1.2.4(e), 2.2.1(f). Check your examination year and the complete specification for the assessment scope.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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