Equivalent of NotImplementedError for fields in Python

Question

In Python 2.x when you want to mark a method as abstract, you can define it like so:

class Base:
    def foo(self):
        raise NotImplementedError(\"Subcl         


        

Answer 1

It seem that this question was open to both instance attributes and class attributes, I'll focus on the first topic only.

So, for instance attributes, an alternate answer to Evan's is to define a mandatory field using pyfields:

from pyfields import field

class Base(object):
    example = field(doc="This should contain an example.")

b = Base()
b.example

yields

pyfields.core.MandatoryFieldInitError: 
   Mandatory field 'example' has not been initialized yet 
   on instance <__main__.Base object at 0x000002C1000C0C18>.

Granted, it does not provide you with the ability to edit the error message by talking about subclasses. But in a way it is more realistic to not talk about subclasses - indeed in python, attributes can be overridden on instances of the base class - not only in subclasses.

Note: I'm the author of pyfields. See documentation for details.

Answer 2

A better way to do this is using Abstract Base Classes:

import abc

class Foo(abc.ABC):

    @property
    @abc.abstractmethod
    def demo_attribute(self):
        raise NotImplementedError

    @abc.abstractmethod
    def demo_method(self):
        raise NotImplementedError

class BadBar(Foo):
    pass

class GoodBar(Foo):

    demo_attribute = 'yes'

    def demo_method(self):
        return self.demo_attribute

bad_bar = BadBar()
# TypeError: Can't instantiate abstract class BadBar \
# with abstract methods demo_attribute, demo_method

good_bar = GoodBar()
# OK

Note that you should still have raise NotImplementedError instead of something like pass, because there is nothing preventing the inheriting class from calling super().demo_method(), and if the abstract demo_method is just pass, this will fail silently.