On Aug 25, 2017, at 12:49 PM, Thomas Kluyver <thomas@kluyver.me.uk> wrote:

Can I gently ask everyone involved to consider whether the notimplemented/error discussion is verging into bikeshedding (http://bikeshed.org/)?

The technical arguments I have seen so far are:
- The exception can include a message
- The return value can't 'bubble up' from the internals of a hook like an exception

I don't think the discussion of semantics is going to go anywhere: they are both reasonable ways for the backend to reply "sorry, Dave, I can't do that".

I don’t think they are both reasonable ways any more than it’s reasonable to do ``raise Return(value)`` instead of ``return value``. The semantics here are important because using exceptions for non-exceptional, non erroneous cases has *always*, in my experience, lead to weirdness [1].

[1] Like for example, StopIteration which was deemed so bad as to need to break backwards compatibility and break consistency with all other uses of exceptions just to handle the weirdness in a saner way. Unfortunately we can’t modify the Python interpreter to fix our weirdness that is going to happen.