GoodKangaroo.py

"""This module contains a code example related to

Think Python, 2nd Edition
by Allen Downey
http://thinkpython2.com

Copyright 2015 Allen Downey

License: http://creativecommons.org/licenses/by/4.0/
"""

from __future__ import print_function, division

"""

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:
    """A Kangaroo is a marsupial."""
    
    def __init__(self, name, contents=[]):
        """Initialize the pouch contents.

        name: string
        contents: initial pouch 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 gets 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.name = name
        self.pouch_contents = contents

    def __init__(self, name, contents=None):
        """Initialize the pouch contents.

        name: string
        contents: initial pouch contents.
        """
        # 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 gets 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.
        self.name = name
        if contents == None:
            contents = []
        self.pouch_contents = contents

    def __str__(self):
        """Return a string representaion of this Kangaroo.
        """
        t = [ self.name + ' has 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):
        """Adds a new item to the pouch contents.

        item: object to be added
        """
        self.pouch_contents.append(item)


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

print(kanga)
print(roo)

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