19.2. Introduction to Inheritance

Recall the Point class from earlier in the book:

class Point: def __init__(self, initX, initY): self.x = initX self.y = initY def distanceFromOrigin(self): return ((self.x ** 2) + (self.y ** 2)) ** 0.5 def __str__(self): return "x=" + str(self.x) + ", y=" + str(self.y) p = Point(7, 6) print(p)

Now, suppose we want to create a class that works like Point in every respect, but also keeps track of a short description for the point. We could create a LabeledPoint class by copying and pasting the definition for Point, changing the name to LabeledPoint, and modifying the class to suit our purposes. However, any time you copy and paste code, keep in mind that you are copying and pasting bugs that may exist in the code. Inheritance provides a way to reuse the definition of Point without having to copy and paste.

We begin like this:

class LabeledPoint(Point): pass

This example defines a class named LabeledPoint that inherits from the Point class. Putting the name Point in parenthesis tells Python that the new class, LabeledPoint, begins with all of the methods defined in its parent, Point. For example, we can instantiate LabeledPoint using the Point constructor, and invoke any Point methds we want to on it:

p = LabeledPoint(7,6)
dist = p.distanceFromOrigin()

Now, let’s refine LabeledPoint so that it holds a label, along with the x and y coordinates:

class LabeledPoint(Point): def __init__(self, initX, initY, label): self.x = initX self.y = initY self.label = label def __str__(self): return "x=" + str(self.x) + ", y=" + str(self.y) + " (" + self.label + ")" labeledPt = LabeledPoint(7,6,"Here") print(labeledPt)

Here, we have redefined two of the methods that LabeledPoint inherits from Point: __init__() and __str__(). This is called overriding. When a child class redefines methods that are defined in its parent, we say that the child overrides the functionality inherited from its parent. When both the parent class and child class have a method with the same name, an invocation of the method on an instance of the child class executes code in the child’s class; an invocation of the method on an instance of the parent class executes code in the parent’s class. For example, consider the following:

pt = Point(5,10) labeledPt = LabeledPoint(7, 6, "Here") ptStr = str(pt) labeledPtStr = str(labeledPt)

In Line 4, the call to str(pt) invokes the __str__() method in Point, because pt refers to an instance of Point. In Line 5, the call to str(labeledPt) invokes the __str__() method in LabeledPoint, because labeledPt refers to an instance of LabeledPoint.

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