19.3. Extending¶
If you compare the code in the __init__ methods of Point and LabeledPoint, you can
see that there is some duplication–the initialization of x and y. We can
eliminate the duplication by having LabeledPoint’s __init__() method invoke
Point’s __init__() method. That way, each class will be responsible for
initializing its own instance variables.
A method in a child class that overrides a method in the parent can invoke
the overridden method using super(), like this:
1 class LabeledPoint(Point):
2
3 def __init__(self, initX, initY, label):
4 super().__init__(initX, initY)
5 self.label = label
In this example, line 4 invokes the __init__() method in Point,
passing the values of initX and initY
to be used in initializing the x and y instance variables.
Here is a complete code listing showing both classes, with a version
of __str__() for LabeledPoint that invokes its parent’s implementation
using super() to avoid duplicating the functionality provided in Point.