[Tutor] Re: Creating subclasses (newbie)

[Andrei]

Adam <adam <at> monkeez.org> writes:

> I have a subclass I want to create - my intuition told me
> that it would be done like this:
> 
> class MainClass:
> 	class SubClass:
> 		code...
> 	subclassinstance = SubClass()
> mainclassinstance = MainClass()

Your intuition is wrong :). A subclass is not a class defined in a different
class, but a class that inherits and builds upon an ancestor class (a little bit
like a child inherits from her parents, but is not identical to them). For
example we know that all spheres have a diameter. We also know that all balls
are spheres and are intended for a certain game. However, not all spheres are
for playing games with (e.g. the sun or one of those glass "lightning" spheres).
So let's express that in classes:

class Sphere(object): # means Spere inherits from object
    def __init__(self, diameter):
        self.diameter = diameter
    def getRadius(self): # all spheres have a radius too
        return self.diameter/2

class Ball(Sphere): 
    # Ball inherits from (or is a subclass of) Sphere
    def __init__(self, diameter, game):
        Sphere.__init__(self, diameter) 
                # initializes the Sphere properties
                # (which Ball obviously also has)
        self.game = game # a Ball is meant to play a certain game

football = Ball(14.0, 'football') # instance of Ball
sun = Sphere(1.4e11) # instance of Sphere

print football.getRadius() 
  # method inherited by Ball from Sphere
  # Subclassing Sphere got us all its functionality
  # for free! If you need more sphere-like objects,
  # like a Planet class, they too will inherit all
  # functionality shared by all Spheres.

print football.game # will print game property of Ball instance.

print sun.game # will give an error message, since Sphere does
               # not inherit anything from Ball (parents don't 
               # inherit anything from their children!)

> But it seems that this isn't going to work. I'm reading a
> couple of Python books, but they don't seem to cover this
> topic very well (I don't see any coding examples).

You should look a bit better :). People on the main Python mailing list have
already pointed you to some tutorials, so read those and all will be clear(er).

Yours,

Andrei