16.10. Inheritance

Another powerful feature of object-oriented programming is the ability to create a new class by extending an existing class. When extending a class, we call the original class the parent class and the new class the child class.

csp-10-2-1: A new class can be created by extending an exisiting class. When extending a class, we call the original class the class and the new class the class.

For this example, we move our PartyAnimal class into its own file. Then, we can ‘import’ the PartyAnimal class in a new file and extend it, as follows:

class PartyAnimal:

    def __init__(self, nam):
        self.name = nam
        print(self.name,'constructed')

    def party(self, x) :
        self.x = x
        self.x = self.x + 1
        print(self.name,'party count',self.x)

from party import PartyAnimal class CricketFan(PartyAnimal): points = 0 def six(self): self.points = self.points + 6 self.party() print(self.name,"points",self.points) s = PartyAnimal("Sally") s.party() j = CricketFan("Jim") j.party() j.six() print(dir(j))

When we define the CricketFan class, we indicate that we are extending the PartyAnimal class. This means that all of the variables (x) and methods (party) from the PartyAnimal class are inherited by the CricketFan class. For example, within the six method in the CricketFan class, we call the party method from the PartyAnimal class.

As the program executes, we create s and j as independent instances of PartyAnimal and CricketFan. The j object has additional capabilities beyond the s object.

Sally constructed
Sally party count 1
Jim constructed
Jim party count 1
Jim party count 2
Jim points 6
['__class__', '__delattr__', ... '__weakref__',
'name', 'party', 'points', 'six', 'x']

In the dir output for the j object (instance of the CricketFan class), we see that it has the attributes and methods of the parent class, as well as the attributes and methods that were added when the class was extended to create the CricketFan class.

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