Abstract methods in Python

Wednesday. April 15, 2020

An abstract method is a method defined inside a base class but without an implementation. The popular way to declare an abstract method in Python is to use NotImplementedError exception:

class Foo:
    def my_abstract_method(self):
        raise NotImplementedError

We declare another class Bar that inherits from Foo and we don’t override the abstract method.

>>> Bar()
<__main__.Bar at 0x7f7c94380828>

>>> Bar().my_abstract_method()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in my_abstract_method
NotImplementedError

A better way of doing this is raising the error when the object is being instantiated using the built-in module abc.

from abc import ABCMeta, abstractmethod

class AFoo(metaclass=ABCMeta):
    @abstractmethod
    def my_abstract_method(self):
        pass

And when we will try to instantiate a class that inherits from AFoo without implementing the abstract method, the error will be raised when we try to instantiate an object.

# Assume that class Bar inherits from AFoo
>>> Bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Bar with abstract methods my_abstract_method

Tudor Plugaru

Going forward, one line at a time.