Python: abstract class __init__

问题:

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?

回答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__

回答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.

回答3:

You should define the methods as abstract as well with the @abc.abstractmethod decorator.

回答4:

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

回答5:

You can implement something like below :

from abc import ABC
class A(ABC):
    def __init__(self, n):
        self.n = n
        super(A,self).__init__()

Instantiating this class would throw an error