Part 3

So let’s add a sheep dog so that we can try and herd the sheep and explore forces of fear as well as attraction.

New kinds of Animal

At the moment we have one new class Animal which represents our sheep, so we’re going to create two new kinds of animal, one for the sheep and one for the sheep dog.

Do you remember when we created the Animal class we said it was a new kind of Actor? So our two new classes Sheep and SheepDog are going to be new kinds of Animal.

First we need to change Animal so that it doesn’t assume everything is a sheep. Change your Animal.__init__ method like so:

class Animal(Actor):

    all = []

    def __init__(self, img):
        super(Animal, self).__init__(img)

So now when we create an Animal we must specify the image we want to use.

Now add this code under your Animal class:

class Sheep(Animal):

    def __init__(self):
        super().__init__('sheep.png')

class SheepDog(Animal):

    def __init__(self):
        super().__init__('dog.png')

Finally, instead of creating new animals, we want to create new Sheep or SheepDogs, so change your code where you have Animal() repeatedly to the following – and see how we’ve used a loop to create lots of sheep:

# Make a flock of sheep
for i in range(15):
    Sheep()

SheepDog()

When you press Run you should see that you how have a sheep dog, although it behaves just like a sheep, so let’s fix that next.

An obedient sheep dog

When you look at the Sheep and SheepDog classes you’ll see that there really is only one difference: the image used. We actually want the sheep dog to behave differently:

• To follow our instructions

• To herd the sheep, or put another way: we want the sheep to move away from the dog.

We can do these things by overriding the default behaviour we’ve created for animals. So…

• Animals wander about, but SheepDogs follow the mouse

• Animals flock together, but Sheep move away from SheepDogs

Add a new method called move to your class SheepDog with this code:

class SheepDog(Animal):

    def __init__(self):
        super().__init__('dog.png')

    def move(self):
        self.x, self.y = pygame.mouse.get_pos()

You’ll also need to add import pygame to the top of your file.

Test this and see what difference it makes.

Run away from the sheep dog

We can now override the attraction_to method so that she Sheep flock towards each other but run away from the sheep dog. Add this new method to your Sheep class:

class Sheep(Animal):

    def __init__(self):
        super().__init__('sheep.png')

    def attraction_to(self, other):
        d = self.distance_to(other)
        if isinstance(other, Sheep):
            # Attraction until we get too close
            return 0.1 * -math.cos(d/40)
        elif isinstance(other, SheepDog):
            # Move away
            return -100/d+0.001

There’s quite a bit going on there, let’s step through it:

• We get the distance between ourselves and the other animal

• We check if the other thing is a Sheep with isinstance, this tests the class against our criteria (we’re testing for Sheep first)

• If it is a sheep then as before we have our herding formula

• Then if not, we test for a SheepDog

• If it is a sheep dog then we have a negagive attraction (a repulsion) which gets stronger the smaller the distance it.

Give it a test and see how it works.

Coming up soon

Let’s set some kind of objective so that the game has a purpose…