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19.4. Reuse Through Composition

Inheritance is not the only way to reuse code. Composition occurs when an object stores a reference to one or more objects in one of its instance variables. The object is thus able to reuse code in the objects it embeds within itself.

For example, our LabeledPoint example could have been implemented as follows:

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

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The first thing to notice about this version of LabeledPoint is that it does not inherit from Point. Instead, its constructor instantiates a Point and stores a reference to it in its point instance variable so that it can be used by the other methods.

Next, notice how the distanceFromOrigin() method reuses the code in Point` by invoking it. We say that ``LabeledPoint’s distanceFromOrigin() delegates its implementation to Point’s implementation.

Finally, notice how the __str__() method also reuses the code in Point by calling str(self.point).

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