Chapter 17 Solutions

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class Point():
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def __str__(self):
        return '({}, {})'.format(self.x, self.y)
    
    def __add__(self, other):
        if isinstance(other, Point):
            return self.add_point(other)
        elif isinstance(other, tuple):
            return self.add_tuple(other)
            
    def add_point(self, other):
        return Point(self.x + other.x, self.y + other.y)
    
    def add_tuple(self, in_tuple):
        return Point(self.x + in_tuple[0], self.y + in_tuple[1])

    def __radd__(self, other):
        return self.__add__(other)


    

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p1 = Point(23, 46)
p2 = Point(1, 2)
print(p1 + p2)


    

    




(24, 48)

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p1 = Point(23, 46)
tup = (10, 20)
print(p1 + tup)


    

    




(33, 66)

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p1 = Point(23, 46)
tup = (10, 20)
print(tup + p1)


    

    




(33, 66)

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# Directly copied from the solutions provided in the book!!!!!
# Ver 1, with poential BUG!
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
print roo


    

    




<__main__.Kangaroo object at 0x104a68d90> with pouch contents:
    'wallet'
    'car keys'
    <__main__.Kangaroo object at 0x104a68d10>
<__main__.Kangaroo object at 0x104a68d10> with pouch contents:
    'wallet'
    'car keys'
    <__main__.Kangaroo object at 0x104a68d10>

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# Ver 2

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


    

    




<__main__.Kangaroo object at 0x1048aa810> with pouch contents:
    'wallet'
    'car keys'
    <__main__.Kangaroo object at 0x1048aa790>

<__main__.Kangaroo object at 0x1048aa790> with pouch contents:

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