[Numpy-discussion] Value based promotion and user DTypes

[Sebastian Berg]

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:
> 
<snip>

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...)

> 
> > 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").

Cheers,

Sebastian


> Cheers,
> Ralf
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