Removing unneeded code, adding data files

code/BadKangaroo.py

@@ -0,0 +1,43 @@
+"""
+
+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.

code/GoodKangaroo.py

@@ -0,0 +1,70 @@
+"""
+
+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