I don't quite understand what you are trying to solve. Here's a related problem: One of the issues with duck typing is that something might look sort of like a duck but isn't really and it would be nice to make it easy to avoid using an object in a "half-baked" manner:

    def playWithDuck(d):
       d.walk()
       d.quack()

if someone calls playWithDuck(dog) then the dog's going to get walked before trying to make the dog quack fails. I'd like to be able to avoid that. I could write:

   def playWithDuck(d):
      d.quack   # raises AttributeError if can't quack
      d.walk()
      d.quack()

but of course that doesn't check that d.quack is a function or even has the right kind of signature. And I can write

    def playWithDuck(d):
      if not isinstance(d, Duck):
    raise TypeError("need a Duck")
      ...

which has it's own problems (I don't need a Duck; I just need something that walks like a duck and quacks like a duck).

Is this the problem you are trying to solve? Or something else?

--- Bruce