1. The dot notation

We are going to learn about a modern programming style called Object-Oriented Programming [OOP]. Before we start writing object-oriented programs, we will first learn how to read and understand the notation used.

1.1. All in a dog’s day

Fido is a dog. During a typical day, he does various actions: he eats, runs, sleeps, etc. Here’s how an object-oriented programmer might write this:

Fido = Dog()
Fido.eats()
Fido.runs()
Fido.sleeps()

In addition, Fido has various qualities or attributes. These are variables, like we have seen before except that they “belong” to Fido. He is tall (for a dog) and his hair is black. Here’s how the programmer might write the same things:

Fido.size = "tall";
Fido.hair_colour = "black";

In the object-oriented language, we have the following:

• Dog is an example of a class of objects.
• Fido is an instance (or particular object) in the Dog class.
• eats(), runs() and sleeps() are methods of the Dog class; ‘methods’ are essentially like ‘functions’ which we saw before (the only difference is that they belong in a given class/object/instance).
• size and hair_colour are attributes of a given instance/object; attributes can take any value that a “normal” variable can take.
• The connection between the attributes or the methods with the object is indicated by a “dot” (”.”) written between them.

Objects can also have other objects that belong to them, each with their own methods or attributes:

Fido.tail.wags()
Fido.tail.type = "bushy";
Fido.left_front_paw.moves()
Fido.head.mouth.teeth.canine.hurts()

Let’s now see how Reeborg uses the “dot” notation.

1.2. A Used Robot gets his name

So far, all the programs we wrote instructing Reeborg to accomplish tasks have been written without using the dot notation. Let’s change this, starting with a simple example.

First, we start by selecting the world Empty which has no robot in it.

Now, you might remember what we said about Reeborg: it is old and faulty ... since it is a Used Robot. [We will learn how to fix it and its friends later.] So, we will create our first instance of a UsedRobot and name it, appropriately, Reeborg! We will then instruct it to take one step.

Try it!

Create a robot and have it take its first step using the following code:

reeborg = UsedRobot()
reeborg.move()

When you are done, try to write a more complicated program, having Reeborg’s name preceding any command given to him.

Important

You may have noticed that I named the robot reeborg with all lower case letters. It is a convention in Python (and many other programming languages) to give a name starting with a lower case letter to instances of a class of objects, reserving names that start with an upper case letter, like UsedRobot, for classes of objects.

However, I will often not follow this convention in naming Reeborg and other robots.

1.3. Many robots

Try this!

Select the world Empty which has no robot in it. Then enter the following code:

reeborg = UsedRobot()
reeborg.move()
erdna = UsedRobot()
erdna.turn_left()
erdna.move()
reeborg.move()

You can add even more robots!

1.4. For the advanced reader

In addition to the dot notation, there is another way to get the value of attributes or methods that belong to an object in Python. Suppose I have a Dog() as above, for which I can have the following:

Fido.size = "tall"
Fido.run()  # is an action that Fido can do

With Python, one can use the built-in function getattr, whose name is meant to remind of “get attribute”, as follows:

how_big = getattr(Fido, "size")    # equivalent to how_big = "tall"
action = getattr(Fido, "run")
action()       # equivalent to Fido.run()

getattr can be very useful in some contexts but its use is overly complicated for what we need to do in Reeborg’s world.