On Fri, Aug 25, 2017 at 9:49 AM, 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 believe Nick also feels that an important advantage of
NotImplementedError is: if a frontend doesn't bother to properly
implement the spec and special case NotImplemented, then you'll end up
eventually getting some obscure error like "'NotImplementedType'
object has no attribute ..." when it tries to treat it as a normal
return value. With NotImplementedError, if the frontend doesn't treat
it specially, the default is to treat it like other exceptions and
show a proper traceback and error message. So lazy frontends give
better UX for NotImplementedError than NotImplemented.

Personally, I don't find the argument about lazy frontends terribly
compelling because if we're assuming that we're hitting some buggy
corner case with code not following the spec, then we should also
consider the case of accidentally bubbled NotImplementedErrors.
Between these two cases, an accidentally bubbled NotImplementedError
causes even more confusing outcomes (the build frontend may silently
start invoking other things! vs. a suboptimal error message), and it's
harder to guard against (both designs require properly written
frontends to contain a few lines of code special casing
NotImplemented/NotImplementedError; only NotImplementedError also
requires all careful *backends* to contain just-in-case try/except
guards).

Another minor point that's made me less happy with NotImplemented is:
originally I thought we could make it a general fact about all the
hooks that returning "NotImplemented" should be treated the same as if
the hook were undefined. (Which is pretty much how it works for
__binop__ methods.) On further consideration though I don't think this
is a good idea. (Rationale: it's not really what we want for
get_build_requires_for_sdist, & if we define future hooks that
normally have no return value then there's a danger of buggy frontends
missing it entirely, & it wouldn't have worked for Nick's suggestion
that build_wheel(build_directory=foo) triggering a NotImplemented
should fall back to build_wheel(build_directory=None), which is gone
from the spec now but suggests that this could cause problems in the
future.) So the bottom line of all this is that if we do go with
NotImplemented, I now think it should only be a defined return value
for get_requires_for_build_sdist and build_sdist, and should have
special "sorry I can't do that Dave" semantics that are different from
e.g. a missing get_requires_for_build_sdist hook. All of which will
work fine, it's just less... aesthetically pleasing.

Personally, I still have a weak preference for NotImplemented over
NotImplementedError, but I don't care enough to have a big fight about
it.

It sounds like Nick and Donald are the only two folks who really have
strong opinions here: can the two of you work something out? Should we
flip a coin?

-n

--
Nathaniel J. Smith -- https://vorpus.org

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