[Numpy-discussion] NEP 30 - Duck Typing for NumPy Arrays - Implementation

[Chris Barker]

OK -- I *finally* got it:

when you pass an arbitrary object into np.asarray(), it will create an
array object scalar with the object in it.

So yes, I can see that you may want to raise a TypeError instead, so that
users don't get an object array  scalar when they wre expecting to get an
array-like object.

So it's probably a good idea to recommend that when a class implements
__dauckarray__ that it also implements __array__, which can either raise an
exception or return and ndarray.

-CHB


On Mon, Sep 16, 2019 at 3:11 PM Chris Barker <chris.barker at noaa.gov> wrote:

> On Mon, Sep 16, 2019 at 2:27 PM Stephan Hoyer <shoyer at gmail.com> wrote:
>
>> On Mon, Sep 16, 2019 at 1:45 PM Peter Andreas Entschev <
>> peter at entschev.com> wrote:
>>
>>> What would be the use case for a duck-array to implement __array__ and
>>> return a NumPy array?
>>
>>
>
>> Dask arrays are a good example. They will want to implement
>> __duck_array__ (or whatever we call it) because they support duck typed
>> versions of NumPy operation. They also (already) implement __array__, so
>> they can converted into NumPy arrays as a fallback. This is convenient for
>> moderately sized dask arrays, e.g., so you can pass one into a matplotlib
>> function.
>>
>
> Exactly.
>
> And I have implemented __array__ in classes that are NOT duck arrays at
> all (an image class, for instance). But I also can see wanting to support
> both:
>
> use me as a duck array
> and
> convert me into a proper numpy array.
>
> OK -- looking again at the NEP, I see this suggested implementation:
>
>     def duckarray(array_like):
>         if hasattr(array_like, '__duckarray__'):
>             return array_like.__duckarray__()
>         return np.asarray(array_like)
>
> So I see the point now, if a user wants a duck array -- they may not want
> to accidentally coerce this object to a real array (potentially expensive).
>
> but in this case, asarray() will only get called (and thus __array__ will
> only get called), if __duckarray__ is not implemented. So the only reason
> to impliment __array__ and raise and Exception is so that users will get
> that exception is the specifically call asarray() -- why should they get
> that??
>
> I'm working on a PR with suggestion for this.
>
> -CHB
>
> --
>
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
> NOAA/NOS/OR&R            (206) 526-6959   voice
> 7600 Sand Point Way NE   (206) 526-6329   fax
> Seattle, WA  98115       (206) 526-6317   main reception
>
> Chris.Barker at noaa.gov
>


-- 

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chris.Barker at noaa.gov
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