"""

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

This program explains and corrects a bug in BadKangaroo.py.
Before reading this, you should try to debug BadKangaroo.

"""

class Kangaroo(object):
    """a Kangaroo is a marsupial"""
    
    def __init__(self, contents=[]):
        # The problem is the default value for contents.
        # Default values get evaluated ONCE, when the function
        # is defined; they don't get evaluated again when the
        # function is called.

        # In this case that means that when __init__ is defined,
        # [] gets evaluated and contents gets a reference to
        # an empty list.

        # After that, every Kangaroo that gets the default
        # value get a reference to THE SAME list.  If any
        # Kangaroo modifies this shared list, they all see
        # the change.

        # The next version of __init__ shows an idiomatic way
        # to avoid this problem.
        self.pouch_contents = contents

    def __init__(self, contents=None):
        # In this version, the default value is None.  When
        # __init__ runs, it checks the value of contents and,
        # if necessary, creates a new empty list.  That way,
        # every Kangaroo that gets the default value get a
        # reference to a different list.

        # As a general rule, you should avoid using a mutable
        # object as a default value, unless you really know
        # what you are doing.
        if contents == None:
            contents = []
        self.pouch_contents = contents

    def __str__(self):
        """return a string representation 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
print ''

print roo