2.4 类

class Foo():
    def __init__(self):
        self.id = 8
        self.value = self.get_value()
    def get_value(self):
        pass
    def should_destroy_earth(self):
        return self.id == 42

class Baz(Foo):
    def get_value(self, some_new_parameter):
        """Since 'get_value' is called from the base class's
        __init__ method and the base class definition doesn't
        take a parameter, trying to create a Baz instance will
        fail.
        """
        pass
class Qux(Foo):
    """We aren't aware of Foo's internals, and we innocently
    create an instance attribute named 'id' and set it to 42.
    This overwrites Foo's id attribute and we inadvertently
    blow up the earth.
    """
    def __init__(self):
        super(Qux, self).__init__()
        self.id = 42
        # No relation to Foo's id, purely coincidental
q = Qux()
b = Baz() # Raises 'TypeError'
q.should_destroy_earth() # returns True
q.id == 42 # returns True

class Foo():
    def __init__(self):
        """Since 'id' is of vital importance to us, we don't
        want a derived class accidentally overwriting it. We'll
        prepend with double underscores to introduce name
        mangling.
        """
        self.__id = 8
        self.value = self.__get_value() # Our 'private copy'
    def get_value(self):
        pass

    def should_destroy_earth(self):
        return self.__id == 42
    # Here, we're storing an 'private copy' of get_value,
    # and assigning it to '__get_value'. Even if a derived
    # class overrides get_value is a way incompatible with
    # ours, we're fine
    __get_value = get_value
class Baz(Foo):
    def get_value(self, some_new_parameter):
            pass
class Qux(Foo):
    def __init__(self):
        """Now when we set 'id' to 42, it's not the same 'id'
        that 'should_destroy_earth' is concerned with. In fact,
        if you inspect a Qux object, you'll find it doesn't
        have an __id attribute. So we can't mistakenly change
        Foo's __id attribute even if we wanted to.
        """
        self.id = 42
        # No relation to Foo's id, purely coincidental
        super(Qux, self).__init__()
q = Qux()
b = Baz() # Works fine now
q.should_destroy_earth() # returns False
q.id == 42 # returns True
with pytest.raises(AttributeError):
    getattr(q, '__id')        

class Point():
    def __init__(self, x, y):
        self.x = x
        self.y = y
p = Point(1, 2)
print (p)
# Prints '<__main__.Point object at 0x91ebd0>'

class Point():
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __str__(self):
        return '{0}, {1}'.format(self.x, self.y)
p = Point(1, 2)
print (p)
# Prints '1, 2'