Python Abstraction: A Guide

The notion of abstraction comes from the ability to distil a complicated system into its component parts or fundamental parts. It involves hiding the complexity and showing only the fundamentals. For example, take a car. A car has several functionalities when you enter inside. It has a steering, a brake, a seat, lights etc. When abstracting a car, you break down these component parts and give them names that you use to describe them as well as their functionalities that makes up the whole, the car.

In python, when you specify a data type as an abstract data type, you are specifying the type of data that is stored, the operations supported on it, and the types of the parameters on the operations. This is called the public interface of the abstract data type. And python has a great deal of latitude in providing the specifications for an interface. In python, interface specification uses duck typing. That means, you don’t check the data types at compile time and there are no formal requirements that abstract classes need to be declared. Instead a programmer would have to assume, when in python, that an object supports a known set of behaviors which if they fail the interpreter will raise an error. Duck typing is attributed to a poet, James Whitcomb Riley, who stated that “when I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.”

Mechanism for supporting abstract data types.

Abstraction and interfaces in python are generally implemented by using abstract classes and interfaces. Formally, python provides an abstract base class (ABC). An abstract base class cannot be instantiated, but it defines one or more common methods that all subclasses of the class must have. You realize an ABC by declaring one or more concrete classes that inherit from the abstract base class while providing implementation of the methods declared by the ABC. To declare a class as an abstract base class, ABC, that class needs to inherit from the abc module, a module that provides formal support for ABCs.

When an ABC needs to be an interface, it needs to just provide the method names without any method body. Also, the method is decorated with @abstractmethod to tell python interpreter to look for implementation of that method in the subclasses or concrete classes inheriting from that ABC.

Let’s give an example of data abstraction in python. Let’s say there is a bank providing three methods of payment - payment by credit card, payment by mobile phone, and payment through social media like whatsapp. The banking app could provide a Payment ABC which is declared such and then provides an abstract method, payment, which needs only be implemented by concrete classes inheriting from this payment class.

Let’s code the Payment class ABC as a python abstract class.

    
from abc import ABC, abstractmethod

class Payment(ABC):

    def print_receipt(self):
        print('Receipt printed for payment', 
                  self.__class__.__name__)

    @abstractmethod
    def payment(self):
        pass 

You can see from the code above that I first imported ABC from the abc module. Then the Payment class inherits ABC class in that module, making it a python abstract class. The Payment class has two methods: a concrete method, print_receipt, that applies to all instances of the class including subclasses and an abstract method, payment, which will have to be implemented by concrete classes inheriting from it. I decorated the abstract method with the @abstractmethod decorator to tell python what to expect. Notice that in the print_receipt method, I made a reference to the class name of the object calling the method so that we can be able to find out which subclass we are printing the receipt for.

Now, let’s write concrete classes that inherit from the Payment class and see how they implement the payment method, abstracting its functionality so they can define their own way of payment.

You can see now that each subclass only needs to implement the method declared as an abstract method. If they don’t, python will give an error. After implementing that class, they behave differently while still being recognized as members of the Payment class.

Whenever an object which implements the abstract method invokes the abstract method, its own implementation is called and executed. That’s how python abstraction works. So, you don’t have to be peering about what is happening under the hood because all you need to know is that mobile payment, credit card payment and whatsapp payment work.

I believe you now have a good working knowledge of how abstraction in python works and can implement your own abstract classes and interfaces. Data abstraction in python allows your class to have several objects doing the same thing in their own distinct way. You need not know how it works but just that it works. Isn’t data abstraction in python beautiful? It’s a cool feature of the language.

In fact, python abstraction is one of the OOP concept every programmer needs to know. I have covered other OOP concepts like inheritance and also polymorphism in other posts.

Happy pythoning.