[Numpy-discussion] Value based promotion and user DTypes
ralf.gommers at gmail.com
Wed Jan 27 12:16:37 EST 2021
On Wed, Jan 27, 2021 at 5:44 PM Sebastian Berg <sebastian at sipsolutions.net>
> On Wed, 2021-01-27 at 10:33 +0100, Ralf Gommers wrote:
> > On Tue, Jan 26, 2021 at 10:21 PM Sebastian Berg <
> > sebastian at sipsolutions.net>
> > wrote:
> Thanks for all the other comments, they are helpful. I am considering
> writing a (hopefully short) NEP, to define the direction of thinking
> here (and clarify what user DTypes can expect). I don't like doing
> that, but the issue turns out to have a lot of traps and confusing
> points. (Our current logic alone is confusing enough...)
Sounds good, thanks.
> > > The other tricky example I have was:
> > >
> > > The following becomes problematic (order does not matter):
> > > uint24 + int16 + uint32 -> int64
> > > <== (uint24 + int16) + (uint24 + uint32) -> int64
> > > <== int32 + uint32 -> int64
> > >
> > > With the addition that `uint24 + int32 -> int48` is defined the
> > > first
> > > could be expected to return `int48`, but actually getting there is
> > > tricky (and my current code will not).
> > >
> > > If promotion result of a user DType with a builtin one, can be a
> > > builtin one, then "ammending" the promotion with things like
> > > `uint24 +
> > > int32 -> int48` can lead to slightly surprising promotion results.
> > > This happens if the result of a promotion with another "category"
> > > (builtin) can be both a larger category or a lower one.
> > >
> > I'm not sure I follow this. If uint24 and int48 both come from the
> > same
> > third-party package, there is still a problem here?
> Yes, at least unless you ask `uint24` to take over all of the work
> (i.e. pass in all DTypes at once).
> So with a binary operator design it is "problematic" (in the sense that
> you have to live with the above result). Of course a binary operator
> base does probably not preclude a more complex design.
> I like a binary operator (it seems much easier to reason about and is a
> common design pattern). But it would be plausible to have an n-ary
> design where you pass all dtypes to each and ask them to handle it
> (similar to `__array_ufunc__`).
> We could even have both (the binary version for most things, but the
> ability to hook into the n-ary "reduction").
I'd say just document it and recommend that if >1 custom dtypes are used,
then the user should (if they really care about the issue you bring up)
determine the output dtype you want via some use of result_type and then
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