[Numpy-discussion] A crazy masked-array thought

Charles R Harris charlesr.harris@gmail....
Fri Apr 27 09:33:20 CDT 2012


On Fri, Apr 27, 2012 at 8:15 AM, Charles R Harris <charlesr.harris@gmail.com
> wrote:

>
>
> On Wed, Apr 25, 2012 at 9:58 AM, Richard Hattersley <rhattersley@gmail.com
> > wrote:
>
>> The masked array discussions have brought up all sorts of interesting
>> topics - too many to usefully list here - but there's one aspect I haven't
>> spotted yet. Perhaps that's because it's flat out wrong, or crazy, or just
>> too awkward to be helpful. But ...
>>
>> Shouldn't masked arrays (MA) be a superclass of the plain-old-array (POA)?
>>
>> In the library I'm working on, the introduction of MAs (via numpy.ma)
>> required us to sweep through the library and make a fair few changes.
>> That's not the sort of thing one would normally expect from the
>> introduction of a subclass.
>>
>> Putting aside the ABI issue, would it help downstream API compatibility
>> if the POA was a subclass of the MA? Code that's expecting/casting-to a POA
>> might continue to work and, where appropriate, could be upgraded in their
>> own time to accept MAs.
>>
>>
> That's a version of the idea that all arrays have masks, just some of them
> have "missing" masks. That construction was mentioned in the thread but I
> can see how one might have missed it. I think it is the right way to do
> things. However, current libraries and such will still need to do some work
> in order to not do the wrong thing when a "real" mask was present. For
> instance, check and raise an error if they can't deal with it.
>

To expand a bit more, this is precisely why the current work on making
masks part of ndarray rather than a subclass was undertaken. There is a
flag that says whether or not the array is masked, but you will still need
to check that flag to see if you are working with an unmasked instance of
ndarray. At the moment the masked version isn't quite completely fused with
ndarrays-classic since the maskedness needs to be specified in the
constructors and such, but what you suggest is actually what we are working
towards.

No matter what is done, current functions and libraries that want to use
masks are going to have to deal with the existence of both masked and
unmasked arrays since the existence of a mask can't be ignored without
risking wrong results.

Chuck
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