+1 for the idea, and the patch LGTM. I was initially a bit confused by the wording of the warning: presumably, we're not going to change Python to make returning an instance of a strict float subclass from __float__ illegal (I don't really see how that would be possible); we're just going to check the return type at the internal call sites to __float__ and raise if we get something other than an exact float. That is, I'd still expect this code to work on future versions of Python: >>> class MyFloat(float): pass ... >>> class A: ... def __float__(self): return MyFloat() ... >>> a = A() >>> a.__float__() 0.0 But these would both become an error in Python 3.7 (say): >>> float(a) 0.0 >>> math.sqrt(a) 0.0 Does that match your thinking? We should probably add issues for checking and fixing other places that __float__ is used internally (like the math module).