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

# example of bad __init__
'''this will give us points with only a z attribute, no x and y
class Point3D(Point):
    def __init__(self, z):
        self.z = z
'''                

# this __init__ calls the superclass constructor appropriately
class Point3D(Point):
    def __init__(self, x,y,z):
        Point.__init__(self,x,y)
        self.z = z