Help with classes

[Hans Nowak]

Kaden wrote:

> I am playing with classes and trying to get the hang of them.  I pretty
> much understand how inheritance and sub/superclassing goes I think.  I
> just am having trouble figuring out how to get different classes to
> communicate with each other.  The best way I can think to explain this is
> to post an example, so here's a nearly verbatum snippet from a tutorial I
> recently read:
> 
> class Basket:
> 
>     def __init__(self,contents=None):
>         self.contents = contents or []
> 
>     def add(self,element):
>         self.contents.append(element)
>         print "You put the", element, "in your basket."
> 
>     def remove(self,element):
>         self.contents.remove(element)
>         print "You remove the", element, "from your basket."
> 
>     def print_me(self):
>         result = ""
>         for element in self.contents:
>             result = result + " " + `element`
>         print "You look in your basket and see:" + result
> 
> class NiceBasket(Basket):
> 
>     def open(self):
>         print "You open your basket."
>         state = "open"
> 
>     def close(self):
>         print "You close your basket."
>         state = "closed"
> 
> Now then, the above class works.  However, I want to be able to add some
> checks, such as checking to see if NiceBasket is closed when you try to
> add soemthing to it.  I can't figure out how to do this.  

By overloading methods. For example, add a new 'add' method to
NiceBasket, that checks, and if the check is successful, it calls
its parents' add method:

    def add(self, element):
        if self.state == "open":
            Basket.add(self, element)
        else:
            print "The basket is not open."

I see that in your open and close methods, you use 'state' as a local
variable. This won't work; to make it visible throughout the instance
of NiceBasket, you should add "self." to it:

     def close(self):
         print "You close your basket."
         self.state = "closed"

Initializing this state variable is probably a good idea too. To 
do so, override the __init__ constructor in NiceBasket:

    def __init__(self, contents=None):
        Basket.__init__(self, contents)
        self.state = "closed"  # or open, depending on what you want

> The only way I could see this working is if the checks were done in the
> Basket class, but that would be the wrong place since NiceBasket is the
> class gives the baskets the ability to be opened.  So any checks would
> have to be in NiceBasket, right?

Indeed, and the code samples above show you how. :-)  Note that, when
you call a method in the same class, you do it like this:

    self.method(args)

but calling a parent's method is done like this:
  
    <parentclass>.method(self, args)

e.g. Basket.add(self, element).

HTH,

--Hans