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6. Object-Oriented Programming


The purpose of this section is not to teach Object-Oriented Programming but simply to give an indication of what can be done using Object-Oriented Programming in Reeborg’s World.

6.1. Object-Oriented Programming

Let’s assume you start with an empty world. You can create a robot at the standard location (x=1, y=1) facing towards the East, and have it take one step with the following program:

reeborg = UsedRobot()

This might explain what we mentioned at the beginning when we introduced Reeborg: it has an oil leak and is broken in many ways. This is not surprising given that it is a UsedRobot and not a NewShinyRobot.

Now, sometimes, we might want to have Reeborg start from a different location, facing in a different way ... and perhaps carrying some tokens (but not other objects). Try the following:

reeborg = UsedRobot(2, 3, 'w', 4)
while reeborg.carries_object("token"):

Instead of 'w', you might want to try 'e', 's', 'n'. I’m sure you can figure out what they mean.

6.2. Multiple robots

Time to try something different. By the way, you may have noticed that there’s no pictures in this section. This is intentional: I want for you to try different things and just give you pointers as to what you can try.

reeborg = UsedRobot(2, 3, 'w', 4)
reeborg.set_model(0)                 # The original! :-)
while reeborg.carries_object("token"):

karel = UsedRobot(5, 5, 'n')

# now for a surprise!

Notice the last move() command? Take note of who moves and I will explain at the very end - provided you are still interested by then.

6.3. Better robots

Try the following:

class BetterRobot(UsedRobot):

    def turn_right(self):

reeborg = BetterRobot()

So far, so good ... but not much of an improvement. We can do better.

6.4. Four important ingredients for designing a better robot

Before I show you the code required to make Reeborg turn right just as easily as it turns left, I need to explain three different implementation details of Reeborg’s World.

First, when you see a robot executing commands, its program has long finished its execution. Each time some specific action is performed, a recording of the state of the entire world is done without anything happening on the screen. Once the program has finished its execution, the recording is played back to you which you then see as an animation. Thus, if we are to design some new commands, we will need to somehow record the state of the world, and do it only once so that it appears to be done in a single step, and not having right turns being three left turns.

Secondly, when you see the oil leak being drawn, it is being drawn from one position to the next, as a straight line segment. It also is drawn is such a way that you can see where Reeborg has made a single turn or three turns, and if it retraced its steps, as lines are drawn at slightly different location depending on Reeborg’s direction of movement.

Third ... well, I’ll start by telling you a little story, and then give you the real explanation.

Reeborg has a brain and a body. It does not make much sense to talk about the orientation of its brain ... but it does when it comes to its body. Thus, its body can be assigned coordinates like x and y as well as an orientation. In Object-Oriented notation, we will refer to this as


instead of the usual


Now, that’s the nice story. And if you are familiar with Object-Oriented programming with Python, you are probably telling yourself that this is a very much unnecessary complication.

The real explanation is much more complicated, and may risk to bore you, but it will be brief.


Python programmers use a convention where variable names that start with an underscore, like _prev_x are meant to indicate that they are “private” and should normally not be changed by another programmer.

Reeborg’s code was first written in Javascript. And you can use Javascript to write programs for Reeborg, just as easily as you can using Python.

Using Brython, I wrote a class that communicates with the Javascript “backend”. If I had written the code the second way (self.x instead of self.body.x), when I would have changed the value of self.x for a move() function ... the object to which self.x would have referred to would no longer be the coordinate of a robot, but rather an integer.

However, by writing the code the way I did, self.body refers to a Javascript object, and self.body.x refers to an attribute of this object. Changing this attribute does not change what object self.body is referring to.

Fourth, orientation is actually an integer taking the values 0 to 3.

So, let’s put these four ingredients together to write a better robot that can properly turn right.

class RepairedRobot(UsedRobot):

    def turn_right(self):

        # save previous values to know from where to start drawing
        self.body._prev_orientation = self.body._orientation
        self.body._prev_x = self.body.x
        self.body._prev_y = self.body.y

        # mimic two previous left turns for prior orientation
        self.body._prev_orientation += 2
        self.body._prev_orientation %= 4

        # do right turn
        self.body._orientation += 3
        self.body._orientation %= 4

        # record the new state of the world only once!

Try it out!

By the way, if you wonder how one could have possibly guessed which names to use for the various attributes, just read on and you will find out how.

6.5. Fixing the leak

Try the following:

r = UsedRobot()

reeborg.body._is_leaky = False
reeborg.body._is_leaky = True
reeborg.body._is_leaky = False
reeborg.body._is_leaky = True

So, this tells us enough to see how to implement a robot whose leak can be fixed at will.

class RepairedRobot(UsedRobot):
    def __init__(self, x=1, y=1, orientation='e', tokens=0, leaky=False):
        super().__init__(x=x, y=y, orientation=orientation, tokens=tokens)
        self.body._is_leaky = leaky

fixed = RepairedRobot(3, 3)
leaky = RepairedRobot(5, 5, leaky=True)


6.6. Facing south

We saw a clumsy way to have Reeborg determine if it was facing South or not. Here’s a better way:

class CompassNeedle(UsedRobot):

    def is_facing_south(self):
        return self.body._orientation == RUR.SOUTH

reeborg = CompassNeedle()
while not reeborg.is_facing_south():

So now you know how to fix Reeborg.


The following is for those that are really curious and not afraid to confront the unknown.

6.7. Exploring the code

Reeborg’s code is on Github. However, you do not need to go there to explore the code as I wrote some convenience functions for you. For example, running the following program:

r = UsedRobot()

dir_js is a Javascript function, understood by Python/Brython, that I wrote to enable you to see an object’s methods and attributes. Right now, it does not tell us much. Here is what I get when I do this:



I use a single letter r for the robot name as this is a very short program and I don’t need a descriptive name.

We do not know if they are methods or attributes. __class__ starts and ends with two underscore characters; this is a convention in the Python world to denote some internal Python code that is mostly reserved for advanced programmers. The other is body. So, we know that r.body is something. Run the following code:

r = UsedRobot()

You should see something like:


which you will likely recognize from the previous explanation.

Note that we don’t see any methods, only attributes. To see the actual methods, we need to switch the language to Javascript (you can do so at the very top of Reeborg’s World.)

6.8. Javascript !??

Remember when you ran this code?

r = UsedRobot()

We are going to do the equivalent with Javascript.

At the very top of the Reeborg’s World window, click on Additional options and select Javascript instead of Python. Then run the following code:

var r = new UsedRobot();

Here is what I see when I do this:


So, nothing that starts and end with a double underscore, and we see body as we had in the Python code, but will also see some familiar methods like at_goal(), move() and many others.

Now we are ready to look at some code.

Do this!

Execute the following Javascript code and look at the printed result.

var r = new UsedRobot();

Make sure the code is exactly as written above. Note that I use view_source instead of dir_js which, as it turns out, would not help me at all in this case.

Based on the result that I see printed,

function () {

my next guess is to execute the following.


After doing so, I see the following:

function (robot){
    "use strict";
    robot._prev_orientation = robot._orientation;
    robot._prev_x = robot.x;
    robot._prev_y = robot.y;
    robot._orientation += 1;  // could have used "++" instead of "+= 1"
    robot._orientation %= 4;
    RUR.control.sound_id = "#turn-sound";
    RUR.rec.record_frame("debug", "RUR.control.turn_left");

So, this is the actual code that makes Reeborg turn left. As mentioned above, you might see something slightly different, so you should really try on your own.

So now you know how to get at the secret code powering Reeborg’s World without having to look for the source code repository.

6.9. About the single move()

Reeborg’s World has been designed right from the start to work with multiple robots AND to make it easier for beginners to write simple programs with only one robot. Robots are actually included in a Javascript Array (similar to a Python list) in the world description and an instruction like move() refers to the zeroth element of this array.

When you start with an empty world, the robot array is empty. As you create robot, they get added, with the first one being the zeroth element. This is why, in the first example above where we have two robots, move() is equivalent to reeborg.move().