Recursion problem

[John Machin]

"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.