The python standard library makes extensive use of duck typing.  Duck typing is a pretty fundamental part of the design of Python, IMO.  Even the ABC module does a bunch of duck typing, rather than requiring strict subclassing or registration.

I think the request is valid, and it is mostly a matter of agreeing on the best way to identify function ducks.  (I agree that Steven's example is intentionally trying to quack like a duck and so is not, IMO, a valid counter argument against using __code__).  I doubt we would make such a change in anything except a feature release, though. 

Let's see what other devs besides Steven and I think.