Programming technique · B3.2.1 · Higher level only

Inheritance

Inheritance allows a specialised child class to reuse and extend a defensible parent-class design. The code is usually short; the important decision is whether the relationship genuinely represents an is-a hierarchy.

Start with the relationship, not the keyword

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Dog is an Animal

A Dog can reuse shared Animal state and behaviour, then add specialised details.

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Human is an Animal

A Human is another specialised Animal type with additional state and behaviour.

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Habitat is not an Animal

An Animal may have a Habitat, but that whole–part relationship is not inheritance.

Parent class

Animal

shared name, energy and behaviour

Child class

Dog

toy and play behaviour

Child class

Human

language and exercise behaviour

Apply the hierarchy in Java

extends establishes the relationship. Each child constructor uses super(...) to construct the Animal part before storing specialised state.

Apply the hierarchy in Python

Put the parent class in parentheses, such as class Dog(Animal). Each child initialiser calls super().__init__(...) before storing specialised state.

Trace construction from parent to child

1new Dog(...)

The Dog constructor begins.

2super(name, energy)

The Animal constructor stores shared state.

3Dog-only field

The child stores specialised state.

4Completed object

The caller can use inherited and Dog-only behaviour.

1Dog(...)

Python creates the object and calls Dog.__init__.

2super().__init__(...)

The Animal initialiser stores shared state.

3Dog-only attribute

The child stores specialised state.

4Completed object

The caller can use inherited and Dog-only behaviour.

Inheritance does not erase access rules

Parent member	Effect in Java subclass code
public	Inherited and accessible through normal visibility rules.
protected	Accessible to subclasses and through package rules.
private	Subclass code cannot name the member directly.
default	Accessible inside the same package, not as a general cross-package permission.

Python naming	Effect in subclass code
`name`	Public by convention and inherited normally.
`_name`	Signals protected/internal use by convention; Python does not enforce it.
`__name`	Name-mangled to reduce accidental access and clashes; use parent operations rather than guessing the mangled name.
No default/package modifier	Python does not reproduce Java package-private access.

Syllabus comparison: you still need to understand Java-style public, private, protected and default access because the standard names them. Python expresses similar design intent mainly through conventions and name mangling rather than enforced modifiers.

Why use inheritance—and when not to

Useful when

the child genuinely satisfies an is-a relationship;
classes share stable state or behaviour;
specialised classes extend rather than duplicate the parent design.

Costs and risks

children become coupled to parent decisions;
a weak hierarchy spreads unsuitable behaviour;
deep hierarchies are harder to trace and maintain.

Challenges Choose one

Choose a challenge that feels appropriate for you. Code heat is only a rough estimate, not a fixed level.

Complete the Animal Hierarchy

Code heat

Challenge ID: PC-T22-C01 · Standards: B3.2.1

Complete the supplied Animal, Dog and Human hierarchy. Store shared name and energy state in Animal, call the parent constructor with super(...), keep specialised fields in the correct subclasses and use inherited public or protected operations rather than directly reaching private parent fields. Create one Dog and one Human, call inherited and specialised behaviour, and trace the final independent state of both objects.

Scaffold available

An Animal parent class branching to Dog and Human child classes.

Inheritance Design and Diagnosis

Code heat

Challenge ID: PC-T22-C02 · Standards: B3.2.1

Repair the supplied Dog subclass by correcting constructor chaining and private-member access. Then add concise comments that justify Dog is-an Animal, reject Habitat is-an Animal, replace it with an Animal has-a Habitat relationship, and explain one reuse benefit and one coupling or maintenance cost of the hierarchy.

Scaffold available

Dog shown as a valid Animal child while Habitat is shown as a separate has-a relationship.

Selected challenge

ID: Language: Code heat: Standards:

Operating system

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Complete the challenge

Write and test your own solution in the folder created above.

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Next boundary: this page reuses parent behaviour but does not yet teach runtime selection of overridden methods through a parent reference. That belongs to polymorphism.