Abstract Base Classes (abc)
Setting the ABCMeta metaclass
Abstract classes are classes that are meant to be inherited but avoid implementing specific methods, leaving behind only method signatures that subclasses must implement.
Abstract classes are useful for defining and enforcing class abstractions at a high level, similar to the concept of interfaces in typed languages, without the need for method implementation.
One conceptual approach to defining an abstract class is to stub out the class methods, and then raise a NotImplementedError if accessed. This prevents children classes from accessing parent methods without overriding them first. Like so:
Creating an abstract class in this way prevents improper usage of methods that are not overriden, and certainly encourages methods to be defined in child classes, but it does not enforce their definition. With the
abc module we can prevent child classes from being instantiated when they fail to override abstract class methods of their parents and ancestors:
It is now possible to simply subclass and override:
Why/How to use ABCMeta and @abstractmethod
Abstract base classes (ABCs) enforce what derived classes implement particular methods from the base class.
To understand how this works and why we should use it, let's take a look at an example that Van Rossum would enjoy. Let's say we have a Base class "MontyPython" with two methods (joke & punchline) that must be implemented by all derived classes.
When we instantiate an object and call it's two methods, we'll get an error (as expected) with the
However, this still allows us to instantiate an object of the ArgumentClinic class without getting an error. In fact we don't get an error until we look for the punchline().
This is avoided by using the Abstract Base Class (ABC) module. Let's see how this works with the same example:
This time when we try to instantiate an object from the incomplete class, we immediately get a TypeError!
In this case, it's easy to complete the class to avoid any TypeErrors:
This time when you instantiate an object it works!