"""

This program is part of an exercise in
Think Python: An Introduction to Software Design
Allen B. Downey

WARNING: this program contains a NASTY bug.  I put
it there on purpose as a debugging exercise, but
you DO NOT want to emulate this example!

"""

class Kangaroo(object):
    """a Kangaroo is a marsupial"""
    
    def __init__(self, contents=[]):
        """initialize the pouch contents; the default value is
        an empty list"""
        self.pouch_contents = contents

    def __str__(self):
        """return a string representaion of this Kangaroo and
        the contents of the pouch, with one item per line"""
        t = [ object.__str__(self) + ' with pouch contents:' ]
        for obj in self.pouch_contents:
            s = '    ' + object.__str__(obj)
            t.append(s)
        return '\n'.join(t)

    def put_in_pouch(self, item):
        """add a new item to the pouch contents"""
        self.pouch_contents.append(item)

kanga = Kangaroo()
roo = Kangaroo()
kanga.put_in_pouch('wallet')
kanga.put_in_pouch('car keys')
kanga.put_in_pouch(roo)

print kanga

# If you run this program as is, it seems to work.
# To see the problem, trying printing roo.