How define constructor implementation for an Abstract Class in Python?

Making the __init__ an abstract method:

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

helps:

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

Python 3 version:

from abc import ABCMeta, abstractmethodclass A(object, metaclass=ABCMeta):    @abstractmethod    def __init__(self, n):        self.n = nif __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):  passa = A(1)  # Will throw exceptionb = B(1)  # Works fine as expected.

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 "<ipython-input-8-3cd318a12eea>", line 1, in <module>    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