[Numpy-discussion] NEP 42 status
Sebastian Berg
sebastian at sipsolutions.net
Wed Mar 17 18:12:32 EDT 2021
On Wed, 2021-03-17 at 07:56 -0500, Lee Johnston wrote:
> I am willing to wait for PR #18398 as I am mainly interested at this
> point
> in the process of developing a new DType and then array coercion and
> casting.
>
> Does _rational_tests.c.src
> <
> https://github.com/numpy/numpy/blob/main/numpy/core/src/umath/_rational_tests.c.src
> >
> illustrate
> the new DType?
>
Thanks for joining the communit call!
The `rational_tests` are still using the old API and unfortunately
there is no great example of the new API, because the API is not public
yet and dealing with "old dtypes" in NumPy obfuscates it a bit.
Let me try to summarize my take-away from discussion and next steps:
As discussed, I think we agreed on the idea of exposing the new API
"experimentally" with the following mechanism:
1. We add a new header, distinct from the normal NumPy headers.
2. This header will use private Python API to achieve:
- Strict version ABI/API requirements. If the code is updated in
NumPy we will increase this version. Possible very often.
A mismatch will cause a strict failure requiring the user to
"keep up" with the NumPy development.
- NumPy will prohibit exporting the public API unless a
`NUMPY_EXPERIMENTAL_DTYPE_API=1` environment variable is set.
This will hopefully prevent the use in production code even if we
make a release.
3. In parallel, I will create a small "toy" DType based on that
experimental API. Probably in a separate repo (in the NumPy
organization?).
Anyone using the API, should expect bugs, crashes and changes for a
while. But hopefully will only require small code modifications when
the API becomes public.
My personal plan for a toy example is currently a "scaled integer".
E.g. a uint8 where you can set a range `[min_double, max_double]` that
it maps to (which makes the DType "parametric").
We discussed some other examples, such as a "modernized" rational
DType, that could be nice as well, lets see...
Units would be a great experiment, but seem a bit complex to me (I
don't know units well though). So to keep it baby steps :) I would aim
for doing the above and then we can experiment on Units together!
Since it came up: I agree that a Python API would be great to have. It
is something I firmly kept on the back-burner... It should not be very
hard (if rudimentary), but unless it would help experiments a lot, I
would tend to leave it on the back-burner for now.
Cheers,
Sebastian
[1] Maybe a `uint8` storage that maps to evenly spaced values on a
parametric range `[double_min, double_max]`. That seems like a good
trade-off in complexity.
> On Tue, Mar 16, 2021 at 4:11 PM Sebastian Berg <
> sebastian at sipsolutions.net>
> wrote:
>
> > On Tue, 2021-03-16 at 13:17 -0500, Lee Johnston wrote:
> > > Is the work on NEP 42 custom DTypes far enough along to
> > > experiment
> > > with?
> > >
> >
> > TL;DR: Its not quite ready, but if we work together I think we
> > could
> > experiment a fair bit. Mainly ufuncs are still limited (though not
> > quite completely missing). The main problem is that we need to
> > find a
> > way to expose the currently private API.
> >
> > I would be happy to discuss this also in a call.
> >
> >
> > ** The long story: **
> >
> > There is one more PR related to casting, for which merge should be
> > around the corner. And which would bring a lot bang to such an
> > experiment:
> >
> > https://github.com/numpy/numpy/pull/18398
> >
> >
> > At that point, the new machinery supports (or is used for):
> >
> > * Array-coercion: `np.array([your_scalar])` or
> > `np.array([1], dtype=your_dtype)`.
> >
> > * Casting (practically full support).
> >
> > * UFuncs do not quite work. But short of writing `np.add(arr1,
> > arr2)`
> > with your DType involved, you can try a whole lot. (see below)
> >
> > * Promotion `np.result_type` should work very soon, but probably
> > isn't
> > is not very relevant anyway until ufuncs are fully implemented.
> >
> > That should allow you to do a lot of good experimentation, but due
> > to
> > the ufunc limitation, maybe not well on "existing" python code.
> >
> >
> > The long story about limitations is:
> >
> > We are missing exposure of the new public API. I think I should be
> > able to provide a solution for this pretty quickly, but it might
> > require working of a NumPy branch. (I will write another email
> > about
> > it, hopefully we can find a better solution.)
> >
> >
> > Limitations for UFuncs: UFuncs are the next big project, so to try
> > it
> > fully you will need some patience, unfortunately.
> >
> > But, there is some good news! You can write most of the "ufunc"
> > already, you just can't "register" it.
> > So what I can already offer you is a "DType-specific UFunc", e.g.:
> >
> > unit_dtype_multiply(np.array([1.], dtype=Float64UnitDType("m")),
> > np.array([2.], dtype=Float64UnitDtype("s")))
> >
> > And get out `np.array([2.], dtype=Float64UnitDtype("m s"))`.
> >
> > But you can't write `np.multiple(arr1, arr2)` or `arr1 * arr2` yet.
> > Both registration and "promotion" logic are missing.
> >
> > I admit promotion may be one of the trickiest things, but trying
> > this a
> > bit might help with getting a clearer picture for promotion as
> > well.
> >
> >
> > The main last limitation is that I did not replace or create
> > "fallback"
> > solutions and/or replacement for the legacy `dtype->f-><slots>`
> > yet.
> > This is not a serious limitation for experimentation, though. It
> > might
> > even make sense to keep some of them around and replace them
> > slowly.
> >
> >
> > And of course, all the small issues/limitations that are not fixed
> > because nobody tried yet...
> >
> >
> >
> > I hope this doesn't scare you away, or at least not for long :/.
> > It
> > could be very useful to start experimentation soon to push things
> > forward a bit quicker. And I really want to have at least an
> > experimental version in NumPy 1.21.
> >
> > Cheers,
> >
> > Sebastian
> >
> >
> > > Lee
> > > _______________________________________________
> > > NumPy-Discussion mailing list
> > > NumPy-Discussion at python.org
> > > https://mail.python.org/mailman/listinfo/numpy-discussion
> >
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