07 Prepare : Reading
Outcomes
At the end of this week, successful students will be able to:
Show fluency in discussing polymorphism.
Write programs that correctly use polymorphism to solve problems.
Preparation Material
For this week, please read the following introduction to Polymorphism:
Then, read this more detailed description of how this works in Python:
Additional Material
Sometimes, we create a base class for the sake of helping to organize and combine common elements of derived classes, yet it doesn't make sense to instantiate the base class on its own. For example, if we were creating classes to model bank accounts, we could imagine having one class for a checking account, and another for a savings account. Because these classes would have things in common, we could imagine creating a base "Account" class. And yet, you can never create a generic account. At the end of the day, you either have a checking account or a savings account.
Abstract Base Classes
A base class that should not be instantiated on its own, but is used to define common elements of derived classes is called an abstract base class.
Often with abstract base classes, we can see that all derived classes should implement a common method, but the logic of this method will be different for each derived class. For example, all bank accounts may have a method to calculate fees, but how this is done for savings accounts could be very different for checking accounts. In this case it may be useful to define this method in the base class to show that it's there, but then we would expect each derived class to override it.
Abstract Methods
For methods that we expect to always be overridden, we can go one step further and force them to be overridden by declaring them as "abstract methods."
In this case, if someone comes along and creates a new type of bank account later, they will not be able to use it (they'll get an error) unless they override all the abstract methods. This guarantees that when we use any type of account we can count on the method being there.
Python 3 Syntax
In Python 3, abstract base classes are defined by inheriting from a class called "ABC". This is done as follows:
from abc import ABC
class Account(ABC):
def __init__(self):
self.name = ""
self.balance = 0.0
Instructor Tips:
Please note that "ABC" is the actual name that you will use, it is not just a made up term like "use class x to do thing y." It stands for Abstract Base Class.
Abstract methods are declared in this abstract base class, by using the abstractmethod decorator before the method as follows:
from abc import ABC
from abc import abstractmethod
class Account(ABC):
def __init__(self):
self.name = ""
self.balance = 0.0
@abstractmethod
def get_fees(self):
pass
Now, all classes that derive from the Account class must implement the get_fees method.
As a side note, be aware that if desired, you can provide a body to the abstract method that could be invoked with the super().get_fees() call in a derived class.