Abstract Classes in Python

Before Python 2.6 there was no explicit way to declare an abstract class. It changed with the abc (Abstract Base Class) module from the standard library.

abc module

abc module allows to enforce that a derived class implements a particular method using a special @abstractmethod decorator on that method.

from abc import ABCMeta, abstractmethod

class Animal:
    __metaclass__ = ABCMeta

    @abstractmethod
    def say_something(self): pass

class Cat(Animal):
    def say_something(self):
        return "Miauuu!"
>>> a = Animal()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Animal with abstract methods say_something

An abstract method can also have an implementation, but it can only be invoked with super from a derived class.

class Animal:
    __metaclass__ = ABCMeta

    @abstractmethod
    def say_something(self):
          return "I'm an animal!"

class Cat(Animal):
    def say_something(self):
        s = super(Cat, self).say_something()
        return "%s - %s" % (s, "Miauuu")
>>> c = Cat()
>>> c.say_something()
"I'm an animal! - Miauuu"

There is more feautres provided by abc module, but they are less common in use than these described in this post. For details check the documentation.

More Pythonic Approach?

Such explicit declaration provided by abc module may be considered not very pythonic. Because of Python's dynamic nature there are few things being checked during compilation, and there is no advanced type checking at that stage. For that reason, we could declare an abstract method by just raising a NotImplementedError.

class Animal:

    def say_something(self):
        raise NotImplementedError()

Additionaly, a class could follow some naming conventions e.g. prefixing a class name with Base or Abstract.

Summary

Despite additional complexity, I find abc module quite useful: it provides a slightly efficient way to communicate the purpose of the code due to its explicitness along with an better flexibility due to possible implementation inside the abstract method.

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