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:

Answer 1

You can override __new__ method to prevent direct instantiation.

class A(object):
    __metaclass__ = ABCMeta

    def __new__(cls, *args, **kwargs):
        if cls is A:
            raise TypeError(
                "TypeError: Can't instantiate abstract class {name} directly".format(name=cls.__name__)
            )
        return object.__new__(cls)

Output:

>>> A()
Traceback (most recent call last):
  File "", line 1, in 
    A()
  File "/Users/ashwini/py/so.py", line 11, in __new__
    "TypeError: Can't instantiate abstract class {name} directly".format(name=cls.__name__)
TypeError: TypeError: Can't instantiate abstract class A directly