Course 1.2Lesson 4 of 5

Is a, or has a?

Two ways to build a class from other classes are inheritance, which says is a, and composition, which says has a, where an object keeps other objects in its attributes. A Dog is an Animal, but a Car is not an Engine, it has an Engine. Composition is more flexible, because the parts can change, and it avoids long family trees. A Garage that has a list of Cars, and a Car that has an Engine, are composition. A rule of thumb is to prefer composition, and to use inheritance only for a clear is a. When a design feels hard, draw the objects and the arrows and read each arrow as is a or has a.

about 25 min1 simulation2 practice questions

Lesson 4 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

tell "is a" from "has a",
build a class that holds other objects,
choose between inheritance and composition, and
read a design as a set of objects and arrows.

Two ways to put classes together

You now know one way to build a class from another: inheritance. There is a second, and it is used even more often: composition. An object simply holds other objects in its attributes.

Two boxes. The green one on the left says is a, inheritance: a Dog is an Animal, written class Dog of Animal. Use it when the child really is a kind of the parent. The orange one on the right says has a, composition: a Car has an Engine, written self dot engine equals Engine. Use it when one thing is made of, or uses, another thing.

Ask one question, and say the sentence aloud.

A Dog is an Animal. Say it: it sounds right. Use inheritance.
A Car has an Engine. A car is not an engine. Use composition: the car keeps an engine in an attribute.

Composition

Predict first. Which cars have a running engine after the first car drives? Then run it.

Car.__init__ makes an Engine and keeps it: self.engine = Engine(power). The car has an engine. A car does not have to know how an engine starts. It asks: self.engine.start().
Garage has a list of cars. total_power goes through the cars and, for each, into its engine: car.engine.power. The dots read like a path: garage, car, engine, power.
After g.cars[0].drive(), only the first engine is running. The second car's engine is a different object, still off.

Each class has one job, and the objects work together. An engine knows about power and starting. A car knows about driving. A garage knows about parking.

Why composition is often better

A clear job for each class. The code of an engine is in Engine, not mixed into Car.
The parts can change. To give a car an electric motor, you make a new class ElectricMotor with the same methods, and put it in the attribute. No family tree to rebuild.
No long chains. Inheritance makes a tight link: a change in a parent can break many children. Composition links by a small set of methods.
It matches the real world. Things are made of things.

Ask the question in words: a circle is a shape (inheritance), and a circle has a centre (composition).

Choosing

Question	Answer: inheritance	Answer: composition
Is the sentence "A is a B" true?	yes	no
Does A use or contain a B?	no	yes
Will A need to swap the B?	no	yes
Do A and B share a lot of code and the same methods?	yes	not necessarily

A famous rule of thumb from experienced programmers: prefer composition, and use inheritance only when the "is a" is clear. Many programs need no inheritance at all.

Reading a design

When you plan a program, draw the objects as boxes and the relations as arrows, and write on each arrow either is a or has a. For a school:

A Student is a Person (inheritance).
A Teacher is a Person (inheritance).
A Classroom has Student objects (composition).
A Student has a list of Mark objects (composition).

If an arrow will not take either label, the design needs another thought.

Common mistakes

Inheriting to reuse code when the "is a" is false. A Stack that inherits from list is a list in disguise, and allows operations that a stack should not.
A long family tree. More than three levels is hard to follow.
A class that reaches deep into others with a long chain: garage.cars[0].engine.parts[2].serial. Give each class a method for what the outside needs.
Sharing one object between two holders by accident (a shared default list).
Forgetting that an attribute holds a reference. Two cars can hold the same engine object.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

Warm-up. Open the problem Is a, or has a? and choose for six pairs.
Build. Open the lab A garage of cars. You build the classes, and the checker tests them.
Stretch. Open the problem Draw the school and write the design as arrows with labels.

Remember these

Inheritance says "is a". Composition says "has a": an object keeps other objects in attributes.
Prefer composition. Use inheritance only for a clear "is a".
Each class should have one job.
Read a design as boxes and arrows, and label each arrow.