Abstract attribute (not property)?

Question

What\'s the best practice to define an abstract instance attribute, but not as a property?

I would like to write something like:

class AbstractFoo(me         


        

Answer 1

If you really want to enforce that a subclass define a given attribute, you can use metaclass. Personally, I think it may be overkill and not very pythonic, but you could do something like this:

 class AbstractFooMeta(type):

     def __call__(cls, *args, **kwargs):
         """Called when you call Foo(*args, **kwargs) """
         obj = type.__call__(cls, *args, **kwargs)
         obj.check_bar()
         return obj


 class AbstractFoo(object):
     __metaclass__ = AbstractFooMeta
     bar = None

     def check_bar(self):
         if self.bar is None:
             raise NotImplementedError('Subclasses must define bar')


 class GoodFoo(AbstractFoo):
     def __init__(self):
         self.bar = 3


 class BadFoo(AbstractFoo):
     def __init__(self):
         pass

Basically the meta class redefine __call__ to make sure check_bar is called after the init on an instance.

GoodFoo()  # ok
BadFoo ()  # yield NotImplementedError