How define constructor implementation for an Abstract Class in Python?

Question

I am trying to declare an abstract class A with a constructor with a default behavior: all subclasses must initialize a member self.n:

from abc import ABCMeta  class A(object):     __metaclass__ = ABCMeta      def __init__(self, n):         self.n = n 

However, I do not want to let the A class be instantiated because, well, it is an abstract class. The problem is, this is actually allowed:

a = A(3) 

This produces no errors, when I would expect it should.

So: how can I define an un-instantiable abstract class while defining a default behavior for the constructor?

Answer 1

Making the __init__ an abstract method:

from abc import ABCMeta, abstractmethod  class A(object):     __metaclass__ = ABCMeta      @abstractmethod     def __init__(self, n):         self.n = n   if __name__ == '__main__':     a = A(3) 

helps:

TypeError: Can't instantiate abstract class A with abstract methods __init__ 

Python 3 version:

from abc import ABCMeta, abstractmethod  class A(object, metaclass=ABCMeta):      @abstractmethod     def __init__(self, n):         self.n = n   if __name__ == '__main__':     a = A(3) 

Works as well:

TypeError: Can't instantiate abstract class A with abstract methods __init__ 

Answer 2

A not so elegant solution can be this:

class A(object):   def __init__(self, n):     if self.__class__ == A:       raise Exception('I am abstract!')     self.n = n 

Usage

class B(A):   pass a = A(1)  # Will throw exception b = B(1)  # Works fine as expected.