Class Relationships and Simple Diagrams

Why it matters

This module has built up several classes — LibraryBook, ReferenceBook, Account, Member, Animal and its subclasses — connected by two kinds of relationship. As a design grows, holding all of it in your head from code alone gets harder. A quick diagram communicates the shape of a design at a glance, the same way pseudocode communicates an algorithm before any Python exists.

Mental model

Every relationship in this module is one of two kinds, and a diagram only needs to show which:

Pseudocodeis-a (inheritance): drawn from specific to general

[LibraryBook]
      ^
      | is-a
      |
[ReferenceBook]

Pseudocodehas-a (composition): drawn from owner to contained object

[Member] --has-a--> [Account]

The direction matters. ReferenceBook is a LibraryBook — every reference book is a library book, specialized to refuse borrowing. The reverse isn’t true: an ordinary LibraryBook isn’t necessarily a ReferenceBook. Member has a Account — a member owns one, but isn’t one itself.

Diagram it

Every relationship built in this module
Classes	Relationship	Diagram
LibraryBook, ReferenceBook	is-a	LibraryBook <- ReferenceBook
Animal, Dog / Cat / Bird	is-a	Animal <- Dog, Cat, Bird (each separately)
Member, Account	has-a	Member --has-a--> Account

Multiple classes can point at the same parent in an is-a diagram — Dog, Cat, and Bird are each their own is-a Animal relationship, drawn separately, not chained to each other.

Check your understanding

Or reveal the answer without checking

Answer:has-a, Car composed of Engine
A car owning an engine, without being a specialized kind of engine, is exactly a has-a relationship — composition, not inheritance.

Or reveal the answer without checking

Answer:is-a only holds one way: the specific class must always be a valid example of the general one, and checking the reverse direction catches an arrow drawn backwards
Checking both directions — does A is-a B, does B is-a A — reveals which one is actually true. Only the direction from specific to general holds; confirming the reverse fails is what catches a backwards arrow.

Practice: warm-up

For each pair, decide whether it’s is-a or has-a, and sketch which direction the relationship points: (1) a SportsCar and a Car, (2) a Playlist and a Song, (3) a Square and a Shape.

Stuck? Reveal one hint at a time.

Hint 1
For each pair, ask: is every example of the first thing also a valid example of the second? If yes, it's is-a.
Hint 2
A playlist contains songs — does that sound like ownership (has-a) or specialization (is-a)?

Reveal the classification

Try the problem yourself before reading this. There is often more than one reasonable approach — treat this as one worked example, not the only correct answer.

SportsCar / Car   -> is-a   (Car <- SportsCar)
Playlist / Song   -> has-a  (Playlist --has-a--> Song)
Square / Shape    -> is-a   (Shape <- Square)

Practice: apply it

A teammate sketched this diagram for a Manager and an Employee:

[Manager]
    ^
    | is-a
    |
[Employee]

Or reveal the answer without checking

Answer:No — it says every Employee is-a Manager, which isn't true; it should be drawn the other way, Employee at the top
This diagram's arrow points from Employee up to Manager, which reads as 'Employee is-a Manager' — backwards. Every manager is an employee, but not every employee is a manager, so Manager is the specific class and belongs at the bottom, pointing up at Employee.

Modification challenge: redraw the diagram with Employee at the top and Manager at the bottom, pointing up at it — matching the direction this lesson’s LibraryBook / ReferenceBook example used.

Summary

A class diagram sketches classes as boxes and relationships as labeled arrows — is-a for inheritance, has-a for composition.
An is-a arrow always points from the specific class to the general one, never the reverse.
Checking a relationship in both directions — does A is-a B, does B is-a A — is what catches an arrow drawn backwards.
Sketching a design before writing code catches a relationship mistake while it’s still a quick fix, not a class hierarchy to untangle.

Key terms

Class diagram