How define constructor implementation for an Abstract Class in Python?

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__

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.