+#!/usr/bin/env python

+# -*- coding: utf-8 -*-

+

+# Think about what will happen if you want to extend the class to calculate

+# the area of different shape? What would you do with the code in the property

+# `total_area` of class `AreaCalculator`?

+

+class Rectangle(object):

+

+    def __init__(self, width, height):

+        self.width = width

+        self.height = height

+

+class AreaCalculator(object):

+

+    def __init__(self, shapes):

+

+        assert isinstance(shapes, list), "`shapes` should be of type `list`."

+        self.shapes = shapes

+

+    @property

+    def total_area(self):

+        total = 0

+        for shape in self.shapes:

+            total += shape.width * shape.height

+

+        return total

+

+def main():

+    shapes = [Rectangle(2, 3), Rectangle(1, 6)]

+    calculator = AreaCalculator(shapes)

+    print "The total area is: ", calculator.total_area

+

+if __name__ == '__main__':

+

+    main()

+#!/usr/bin/env python

+# -*- coding: utf-8 -*-

+# This is a reimplementation of `python_code.bad.open_close` module which

+# abides by the open/close principle. You can see that it will be easy to

+# extend the functionality of `AreaCalculator` in this module whereas it

+# is not in `python_code.bad.open_close` module.

+

+from abc import ABCMeta, abstractproperty

+

+class Shape(object):

+    __metaclass__ = ABCMeta

+

+    @abstractproperty

+    def area(self):

+        pass

+

+class Rectangle(Shape):

+

+    def __init__(self, width, height):

+        self.width = width

+        self.height = height

+

+    @property

+    def area(self):

+        return self.width * self.height

+

+class AreaCalculator(object):

+

+    def __init__(self, shapes):

+        self.shapes = shapes

+

+    @property

+    def total_area(self):

+        total = 0

+        for shape in self.shapes:

+            total += shape.area

+        return total

+

+# Note that if we want to extend the functionality of AreaCalculator to support calculating

+# area of different shape, we only need to define new subtype of `Shape` and leave other code

+# alone without modify them. That is the key of open/close principle.

+

+def main():

+    shapes = [Rectangle(1, 6), Rectangle(2, 3)]

+    calculator = AreaCalculator(shapes)

+

+    print "The total area is: ", calculator.total_area

+

+if __name__ == '__main__':

+    main()