4. Many right turns

Select the world Empty which has no robot in it. Next define the following function:

def turn_right(robot):
    robot.turn_left()
    robot.turn_left()
    robot.turn_left()

Note how we pass the variable robot as an argument to the function and how we use it inside the function; this variable will be the robot’s name.

After defining this function, you can use it as follows:

Reeborg = UsedRobot()
Erdna = UsedRobot(4, 3)  # creating a robot at a different location!

Reeborg.move()
turn_right(Reeborg)
Erdna.turn_left()
turn_right(Erdna)

Try it!

Define turn_right and use it with a program like the one written just above.

This works ... but it does not look right since the function that is used to make the robots turn right uses a different syntax from the method that is used to make them turn left. There has to be a better way...

4.1. Derived classes

UsedRobot is an example of a Python class. You can think of a “class” as a collection of objects that have common methods (functions) and attributes. What I am going to show you now is to create a new class that inherits from an existing one. After having done that, we’ll see how we can create new classes that do more than simply inheriting from existing ones.

Try this!

Try the following code:

class UsedRobotClone(UsedRobot):
    pass

Reeborg = UsedRobotClone()
Erdna = UsedRobotClone(4, 3)

# As before, we define a function to make right turns

def turn_right(robot):
    robot.turn_left()
    robot.turn_left()
    robot.turn_left()

Reeborg.move()
turn_right(Reeborg)
Erdna.turn_left()
turn_right(Erdna)

It should do the exact same thing as the previous example, even though we use a different class to create new robots.

pass is a Python keyword that means “do nothing”. It is inserted when we do not want to do anything special but need something so that the block structure (indentation) is interpreted correctly by Python.

Time to do a little be more than just creating a clone of the existing class.

4.2. Designing a new class

I will show you first how we can fix our robot so that it knows how to turn right, and explain what I did afterwords.

class RepairedRobot(UsedRobot):
    def turn_right( this_RepairedRobot ):
        for i in range(3):
            this_RepairedRobot.turn_left()

Here’s how we can then use this new class of objects:

new_Reeborg = RepairedRobot()
new_Erdna = RepairedRobot(4, 3)

new_Reeborg.turn_left()    # as before
new_Reeborg.turn_right()   # new method!

new_Erdna.turn_right()     # this one works too!

Try it!

Try the above code and make sure it works. However, you might want to choose a slightly shorter name than this_RepairedRobot.

4.2.1. Explanation

The Python keyword class indicates that we are going to define a new type of function, one that creates objects. What follows class is RepairedRobot(UsedRobot). RepairedRobot is the name of our new class; by writing UsedRobot between the parentheses, we ensure that the new class RepairedRobot inherits all the methods and attributes that UsedRobot had. Thus, when we write:

new_Reeborg = RepairedRobot()

we create a new robot “named” new_Reeborg which can do (at least all) the same things that the old:

Reeborg = UsedRobot()

could do.

Next, inside the new class, as indicated by the indented block, we define a new method, turn_right(). By defining it inside the class, we take the first step to insure that all the robots that are created by calling RepairedRobot() will be able to turn right!

The second step that is required is to tell Python that the method will “belong” to the particular object that has been created. To do so, we used above the variable this_RepairedRobot which will refer to new_Reeborg, new_Erdna, etc., depending on what object is created. When we write:

new_Reeborg = RepairedRobot()

Python creates a new instance of the class RepairedRobot and defines all the methods, effectively replacing the first argument of the method (this_RepairedRobot) by the name of the instance (new_Reeborg).

Now, this_RepairedRobot is rather a long name to type. By convention, another variable name is used: self. Thus, to follow the normal convention, I should have written:

class RepairedRobot(UsedRobot):
    def turn_right(self):
        for i in range(3):
            self.turn_left()

Important

It is extremely important that you try the code for the RepairedRobot class, either using self or this_RepairedRobot before moving on to the next lesson.