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:

 class LabeledPoint(Point):

     def __init__(self, initX, initY, label):
         super().__init__(initX, initY)
         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.

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