[Numpy-discussion] missing data discussion round 2

[Mark Wiebe]

On Wed, Jun 29, 2011 at 9:35 AM, Dag Sverre Seljebotn <
d.s.seljebotn at astro.uio.no> wrote:

> On 06/29/2011 03:45 PM, Matthew Brett wrote:
> > Hi,
> >
> > On Wed, Jun 29, 2011 at 12:39 AM, Mark Wiebe<mwwiebe at gmail.com>  wrote:
> >> On Tue, Jun 28, 2011 at 5:20 PM, Matthew Brett<matthew.brett at gmail.com>
> >> wrote:
> >>>
> >>> Hi,
> >>>
> >>> On Tue, Jun 28, 2011 at 4:06 PM, Nathaniel Smith<njs at pobox.com>
>  wrote:
> >>> ...
> >>>> (You might think, what difference does it make if you *can* unmask an
> >>>> item? Us missing data folks could just ignore this feature. But:
> >>>> whatever we end up implementing is something that I will have to
> >>>> explain over and over to different people, most of them not
> >>>> particularly sophisticated programmers. And there's just no sensible
> >>>> way to explain this idea that if you store some particular value, then
> >>>> it replaces the old value, but if you store NA, then the old value is
> >>>> still there.
> >>>
> >>> Ouch - yes.  No question, that is difficult to explain.   Well, I
> >>> think the explanation might go like this:
> >>>
> >>> "Ah, yes, well, that's because in fact numpy records missing values by
> >>> using a 'mask'.   So when you say `a[3] = np.NA', what you mean is,
> >>> 'a._mask = np.ones(a.shape, np.dtype(bool); a._mask[3] = False`"
> >>>
> >>> Is that fair?
> >>
> >> My favorite way of explaining it would be to have a grid of numbers
> written
> >> on paper, then have several cardboards with holes poked in them in
> different
> >> configurations. Placing these cardboard masks in front of the grid would
> >> show different sets of non-missing data, without affecting the values
> stored
> >> on the paper behind them.
> >
> > Right - but here of course you are trying to explain the mask, and
> > this is Nathaniel's point, that in order to explain NAs, you have to
> > explain masks, and so, even at a basic level, the fusion of the two
> > ideas is obvious, and already confusing.  I mean this:
> >
> > a[3] = np.NA
> >
> > "Oh, so you just set the a[3] value to have some missing value code?"
> >
> > "Ah - no - in fact what I did was set a associated mask in position
> > a[3] so that you can't any longer see the previous value of a[3]"
> >
> > "Huh.  You mean I have a mask for every single value in order to be
> > able to blank out a[3]?  It looks like an assignment.  I mean, it
> > looks just like a[3] = 4.  But I guess it isn't?"
> >
> > "Er..."
> >
> > I think Nathaniel's point is a very good one - these are separate
> > ideas, np.NA and np.IGNORE, and a joint implementation is bound to
> > draw them together in the mind of the user.    Apart from anything
> > else, the user has to know that, if they want a single NA value in an
> > array, they have to add a mask size array.shape in bytes.  They have
> > to know then, that NA is implemented by masking, and then the 'NA for
> > free by adding masking' idea breaks down and starts to feel like a
> > kludge.
> >
> > The counter argument is of course that, in time, the implementation of
> > NA with masking will seem as obvious and intuitive, as, say,
> > broadcasting, and that we are just reacting from lack of experience
> > with the new API.
>
> However, no matter how used we get to this, people coming from almost
> any other tool (in particular R) will keep think it is
> counter-intuitive. Why set up a major semantic incompatability that
> people then have to overcome in order to start using NumPy.
>

I'm not aware of a semantic incompatibility. I believe R doesn't support
views like NumPy does, so the things you have to do to see masking semantics
aren't even possible in R.

I really don't see what's wrong with some more explicit API like
> a.mask[3] = True. "Explicit is better than implicit".
>

I agree, but initial feedback was that the way R deals with NA values is
very nice, and I've come to agree that it's worth emulating.

-Mark


>
> Dag Sverre
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