On Thu, May 25, 2017 at 10:49 AM, Mark Shannon <mark@hotpy.org> wrote:
I really like this PEP in general. I think this brings the type system for type-hints closer to Python semantics.

Thank you.
But there are a few points I disagree with.
I don't think Protocol types should be tied to ABCs. It just makes things more complex with no obvious benefit.

There are backwards compatibility benefits -- we could make e.g. Sequence a Protocol in Python 3.7 and it would be possible to write code that inherits from Sequence and works in Python 3.6 and 3.7. For this to work we need some support for non-abstract methods in Protocols.
I also think all references to 'isinstance' and 'issubclass' should be removed. Type-hints should not have runtime semantics, beyond those that they have as classes.

Again, backwards compatibility.
In fact, there is no need for protocol types to be classes at all.

That's pretty much a separate discussion (see https://github.com/python/typing/issues/432). 

--Guido van Rossum (python.org/~guido)