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.
Before you keep reading...
Making great stuff takes time and $$. If you appreciate the book you are reading now and want to keep quality materials free for other students please consider a donation to Runestone Academy. We ask that you consider a $10 donation, but if you can give more thats great, if $10 is too much for your budget we would be happy with whatever you can afford as a show of support.
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)
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.