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("Subclasses should implement this!")

Then if you forget to override it, you get a nice reminder exception. Is there an equivalent way to mark a field as abstract? Or is stating it in the class docstring all you can do?

At first I thought I could set the field to NotImplemented, but when I looked up what it’s actually for (rich comparisons) it seemed abusive.

Answer 1

Yes, you can. Use the @property decorator. For instance, if you have a field called “example” then can’t you do something like this:

class Base(object):

    @property
    def example(self):
        raise NotImplementedError("Subclasses should implement this!")

Running the following produces a NotImplementedError just like you want.

b = Base()
print b.example