Recursion problem

[Dennis Peterson]

Thanks for the help, John, I'll follow your advice. All the oop I've done
until now was C++, which has no class-level data, and thinking in those
terms made this seem very strange.
-Dennis

"John Machin" <sjmachin at lexicon.net> wrote in message
news:c76ff6fc.0205190444.204fd65e at posting.google.com...
> "Dennis Peterson" <denpeterson at yahoo.com> wrote in message
news:<ac7d2u02vgg at enews2.newsguy.com>...
> > I'm trying to implement a basic Composite pattern. In the following
code, I
> > expect c.getData() and d.getData() to both return "hellogoodbye".
Instead,
> > on d.getData() I get stacktrace printing "y += i.getData()" repeatedly
until
> > recursion depth exceeded. Why?
> >
> > I'm new to Python, running latest Windows version just downloaded.
> >
> > class Simple:
> >     def __init__(self,x):
> >         self.data = x
> >     def getData(self):
> >         return self.data
> >
> > class Compound:
> >     data = []
>
> This "data" is a *CLASS*-level attribute, shared by all instances of
> the class.
> You should have an __init__() similar to the other class, to set
> self.data to [].
>
> >     def getData(self):
> >         y = ""
> >         for i in self.data:
> >             y += i.getData()
> >         return y
> >
> > def test():
> >     a = Simple("hello")
> >     b = Simple("goodbye")
> >     c = Compound()
> >     c.data.append(a)
> >     c.data.append(b)
>
> Insert a print statement here ... print c.data
> >     print c.getData()
> >     d = Compound()
> >     d.data.append(c)
>
> Now for the big surprise:
>       print d.data
> *AND*
>       print c.data
> Just like the man said, c.data and d.data are the same thing ... and
> uh-oh, that means c contains a reference to itself!
> >     print d.getData()
>
> d contains c, which contains c, so you start emulating the mythical
> ooloo bird, until saved by the recursion limit.
>
> *HINT* Unless you expect your code to work correctly first time or if
> not that kind strangers will always do your debugging for you, learn
> to put in print statements to give yourself a bit of a clue as to what
> is going wrong. You seem to have jumped right in to the OO aspects of
> Puthon; perhaps you might like to write yourself a mixin class that
> supplies a generic dump() method for debugging instances of classes
> that inherit from it.