20.3. Call a Parent Method

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 call (execute) Point’s __init__() method. That way, each class will be responsible for initializing its own instance variables.

All objects keep a reference to the class that created them and every class has a reference to its parent class. You can get an object that represents the parent class using super() and then call a method on that object.

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.

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

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Finish the code for the Student class below. In the __init__ method for Student call the __init__ method for Person and pass in a first and last name. Then also set the id. In the __str__ method for Student return the id in the form (id: actual_id,) followed by the string returned from the __str__ method in Person. For example, if s = Student("Abby", "Lane", 1) then print(s) should print “id: 1, name: Abby Lane”.

class Person: def __init__(self, first, last): self.first = first self.last = last def __str__(self): return f"name: {self.first} {self.last}" class Student(Person): def __init__(self, first, last, id): def __str__(self): s = Student("Abby", "Lane", 1) print(s) ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): s = Student("Abby", "Lane", 1) self.assertEqual(s.__str__(),"id: 1, name: Abby Lane","Test of Abby Lane with id of 1") s = Student("Sam", "Brown", 2) self.assertEqual(s.__str__(),"id: 2, name: Sam Brown","Test of Sam Brown with id of 2") s = Student("Julia", "Pai", 3) self.assertEqual(s.__str__(),"id: 3, name: Julia Pai","Test of Julia Pai with id of 3") myTests().main()

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