an oop question

[Chris Angelico]

On Mon, 31 Oct 2022 at 14:38, Julieta Shem <jshem at yaxenu.org> wrote:
>
> Chris Angelico <rosuav at gmail.com> writes:
>
> > The most straight-forward way to represent this concept in an
> > object-oriented way is subclassing.
> >
> > class Stack:
> >     ... # put whatever code is common here
> >
> > class Empty(Stack):
> >     ... # put Empty-specific code here, possibly overriding Stack methods
> >
> > class Pair(Stack):
> >    ... # ditto, overriding or augmenting as needed
> >
> > This way, everything is an instance of Stack, but they are still
> > distinct types for when you need to distinguish.
>
> Can you provide a small example?  I can't see what you mean, but it
> seems interesting.

Sure. The easiest way would be to take your existing Empty and Pair
classes, have them subclass Stack, and don't bother putting any code
at all into Stack. Then construct an Empty and a few Pairs, and what
you'll see is that all of them are also instances of Stack.

After that, it's really a question of what you expect to be able to do
with either an Empty or a Pair. Anything that should be possible with
both types (that is, anything that should be possible with either
variant of Stack) should get moved into the Stack type, while anything
that is specific to one or the other stays in its own class. So here's
a very very simple example:

class Stack:
    def prepend(self, other):
        return Pair(other, self)

class Empty(Stack):
    def is_last(self):
        return True
    def get_current(self):
        raise ValueError("Stack empty")
    def get_next(self):
        raise ValueError("Stack empty")

class Pair(Stack):
    def __init__(self, item1, item2):
        self.item1 = item1
        self.item2 = item2
    def get_current(self):
        return self.item1
    def get_next(self):
        return self.item2
    def is_last(self):
        return isinstance(self.item2, Empty)

With this setup, you can build a stack by prepending items onto an
Empty endpoint, and can iterate over it with the get_current and
get_next methods. (Making this actually iterable, so that it works
with a Python 'for' loop, would be a good exercise.)

In this example, there isn't much code in the Stack class. But you
could easily add more, and it would apply to both Empty and Pair.

ChrisA