adds shapes

learning_python3.md

@@ -886,6 +886,7 @@ def pretty_print(msg, decorator="#"):
 	print("{} {} {}".format(decorator, msg, decorator))
 	print(decorator * line_len)
 
+
 pretty_print("Wouter")
 pretty_print("Python rules!")
 pretty_print("Alice", "-")
@@ -1939,6 +1940,7 @@ class Animal(object):
         else:
             print("{} growls in {}'s face".format(self.name, person))
 
+
 dog_1 = Animal("bobby", 4, "dave")
 dog_2 = Animal("dianne", 4, "alice")
 dog_3 = Animal("rex", 3, "dave")
@@ -2041,11 +2043,76 @@ for animal in all_animals:
 🏃 Try it
 ---
 
-Take the additional class yyou made, such as vehicle and do some inheritance.
+Take the additional class you made, such as vehicle and do some inheritance.
 For vehicles you could create a base class with a brand and number of wheels and seats.
 Then inherit for a bus, bike, car, cycle, etc.
 
-**TODO shapes and surfaces**
+## A *practical* example
+
+```python
+import math
+
+
+class Shape(object):
+    def __init__(self):
+        self.area = 0
+
+    def __str__(self):
+        return "I'm a {} with an area of {:.2f} cm2".format(
+                self.__class__.__name__,
+                self.area,
+                )
+    
+    def __add__(self, other):
+        combi_shape = Shape()
+        combi_shape.area = self.area + other.area
+        return combi_shape
+
+    def __sub__(self, other):
+        combi_shape = Shape()
+        combi_shape.area = self.area - other.area
+        return combi_shape
+
+    def __div__(self, value):
+        combi_shape = Shape()
+        combi_shape.area = self.area / value
+        return combi_shape
+
+
+class Square(Shape):
+    def __init__(self, width):
+        Shape.__init__(self)
+        self.area = width * width
+
+
+class Rectangle(Shape):
+    def __init__(self, width, height):
+        Shape.__init__(self)
+        self.area = width * height
+
+
+class Triangle(Shape):
+    def __init__(self, base, height):
+        Shape.__init__(self)
+        self.area = (self.width * self.height) / 2
+
+
+class Circle(Shape):
+    def __init__(self, radius):
+        Shape.__init__(self)
+        self.area = math.pi * math.pow(radius, 2)
+
+
+if __name__ == "__main__":
+    t = Square(4)
+    r = Rectangle(4, 6)
+    c = Circle(1.6)
+    print(t)
+    print(r)
+    print(c)
+    combi = t + r - c
+    print(combi)
+```
 
 ## Now some *practical* improvements