[Numpy-discussion] in the NA discussion, what can we agree on?

Lluís xscript@gmx....
Thu Nov 3 09:28:14 CDT 2011


Nathaniel Smith writes:

> 4) There is consensus that whatever approach is taken, there should be
> a quick and convenient way to identify values that are MISSING,
> IGNORED, or both. (E.g., functions is_MISSING, is_IGNORED,
> is_MISSING_or_IGNORED, or some equivalent.)

Well, maybe it's too low level, but I'd rather decouple the two concepts into
two orthogonal properties that can be composed:

* Destructiveness: whether the previous data value is lost whenever you assign a
  "special" value.

* Propagation: whether any of these "special" values is propagated or just
  skipped when performing computations.

I think we can all agree on the definition of these two properties (where
bit-patters are destructive and masks are non-destructive), so I'd say that the
first discussion is establishing whether to expose them as separate properties
or just expose specific combinations of them:

* MISSING: destructive + propagating
* IGNORED: non-destructive + non-propagating

For example, it makes sense to me to have non-destructive + propagating.


If we take this road, then the next points to discuss should probably be how
these combinations are expressed:

* At the array level: all special values behave the same in a specific array,
  given its properties (e.g., all of them are destructive+propagating).

* At the value level: each special value conveys a specific combination of the
  aforementioned properties (e.g., assigning A is destructive+propagating and
  assigning B is non-destructive+non-propagating).

* Hybrid: e.g., all special values are destructive, but propagation depends on
  the specific special value.

I think this last decision is crucial, as it will have a direct impact on
performance, numpy code maintainability and 3rd party interface simplicity.


Lluis

-- 
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
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