Writing your first class

What you will be able to do

After this lesson you can

• write a class with __init__ and methods,
• make objects from it and read their attributes,
• explain what self is, and
• find the two usual mistakes with self.

A blueprint for a dog

Imagine that you run a pet shop, and you keep a record of every dog: its name and its age. You also want each dog to be able to bark, and to have a birthday. You could keep a dictionary for each dog. A class does it more neatly.

Predict first: what will the last lines show? Then run it.

Now the objects.

• rex = Dog("Rex", 3) makes an object. You call the class like a function. Python makes a new empty object, runs __init__ with that object as self, and gives the object back. We say that rex is an instance of Dog.
• rex.name and rex.age read the attributes. rex.bark() calls the method.
• After rex.birthday(), rex.age is 4, but bella.age is still 5. Each object has its own data. The class is shared. The data is not.

What is self?

The word self confuses everybody at first. Here is the whole story. When you write rex.bark(), Python does Dog.bark(rex): it calls the method of the class, and passes the object rex as the first argument. Inside the method, that first parameter is called self. So self is simply the object that the method was called on.

You can call the first parameter anything. But always call it self. Every Python programmer expects it.

self is the object that the method was called on. Inside area(), self.width is the width of that one rectangle.

The two usual mistakes

Mistake 1: leaving out self in the method.

Mistake 2: forgetting self. in front of an attribute.

A rule to keep: an attribute is always written with self. in front, inside the class. A bare name inside a method is a local variable.

Common mistakes

• No self in the def line. A TypeError about arguments.
• No self. before an attribute. An AttributeError.
• Calling Dog without brackets. rex = Dog does not make a dog. It gives another name to the class.
• Forgetting the arguments of __init__. Dog() without a name is a TypeError.
• Writing __init__ with one underscore, or the wrong spelling. Python does not run it.
• A class name in small letters. It works, but it breaks a habit that helps readers.

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:

1. Warm-up. Open the lab Make a Rectangle. You write a class with two attributes and two methods, and the checker tries it.
2. Build. Open the problem Fix the class and the problem Point and distance.
3. Stretch. Open the problem What does self hold? and trace three calls.

Remember these

• class Name: and methods inside. __init__(self, ...) sets up each new object.
• self.attribute = value makes data that belongs to one object.
• Name(arguments) makes an object. Each object has its own data.
• self is the object that the method was called on, and it is always the first parameter.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does Python run when you write Dog("Rex", 3)?

2. What happens if bark is written def bark():?

3. What is wrong with name = name inside __init__?