On 2018-04-14 23:14, Guido van Rossum wrote:

That actually sounds like a pretty big problem. I'm sure there is lots of code that doesn't *just* duck-type nor calls inspect but uses isinstance() to decide how to extract the desired information.

I have been thinking about this some more... One solution to improve backwards compatibility would be to duplicate some classes. For example, make a separate class for bound methods in extension types, which would be literally a duplicate of the existing types.MethodType class (possibly with a different name). In other words, a bound method of an extension type would work exactly the same way as an existing bound method but it would artificially be a different class for the benefit of non-duck-typing.