On Fri, May 15, 2020 at 5:45 PM Andrew Barnert <abarn...@yahoo.com> wrote:
> On May 15, 2020, at 13:03, Christopher Barker <python...@gmail.com> wrote: > > > > Taking all that into account, if we want to add "something" to Sequence > behavior (in this case a sequence_view object), then adding a dunder is > really the only option -- you'd need a really compelling reason to add a > Sequence method, and since there are quite a few folks that think that's > the wrong approach anyway, we don't have a compelling reason. > > > > So IF a sequence_view is to be added, then a dunder is really the only > option. > > Once you go with a separate view-creating function (or type), do we even > need the dunder? > Indeed -- maybe not. We'd need a dunder if we wanted to make it an "official" part of the Sequence protocol/ABC, but as you point out there may be no need to do that at all. Hmm, more thought needed. -CHB > > I’m pretty sure a generic slice-view-wrapper (that just does index > arithmetic and delegates) will work correctly on every sequence type. I > won’t promise that the one I posted early in this thread does, of course, > and obviously we need a bit more proof than “I’m pretty sure…”, but can > anyone think of a way a Sequence could legally work that would break this? > > And I can’t think of any custom features a Sequence might want add to its > view slices (or its view-slice-making wrapper). > > I can definitely see how a custom wrapper for list and tuple could be > faster, and imagine how real life code could use it often enough that this > matters. But if it’s just list and tuple, CPython’s already full of > builtins that fast-path on list and tuple, and there’s no reason this one > can’t do the same thing. > > So, it seems like it only needs a dunder if there are likely to be > third-party classes that can do view-slicing significantly faster than a > generic view-slicer, and are used in code where it’s likely to matter. Can > anyone think of such a case? (At first numpy seems like an obvious answer. > Arrays aren’t Sequences, but I think as long as the wrapper doesn’t > actually type-check that at __new__ time they’d work anyway. But why would > anyone, especially when they care about speed, use a generic viewslice > function on a numpy array instead of just using numpy’s own view slicing?) > > It seems like a dunder is something that could be added as a refinement in > the next Python version, if it turns out to be needed. If so, then, unless > we have an example in advance to disprove the YAGNI presumption, why not > just do it without the dunder? > > > -- Christopher Barker, PhD Python Language Consulting - Teaching - Scientific Software Development - Desktop GUI and Web Development - wxPython, numpy, scipy, Cython
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