[Python-Dev] PyObject_RichCompareBool identity shortcut

[Guido van Rossum]

On Wed, Apr 27, 2011 at 9:15 PM, Alexander Belopolsky
<alexander.belopolsky at gmail.com> wrote:
> On Wed, Apr 27, 2011 at 2:48 PM, Robert Kern <robert.kern at gmail.com> wrote:
> ..
>> I suspect most of us would oppose changing it on general
>> backwards-compatibility grounds rather than actually *liking* the current
>> behavior. If the behavior changed with Python floats, we'd have to mull over
>> whether we try to match that behavior with our scalar types (one of which
>> subclasses from float) and our arrays. We would be either incompatible with
>> Python or C, and we'd probably end up choosing Python to diverge from. It
>> would make a mess, honestly. We already have to explain why equality is
>> funky for arrays (arr1 == arr2 is a rich comparison that gives an array, not
>> a bool, so we can't do containment tests for lists of arrays), so NaN is
>> pretty easy to explain afterward.
>
> Most NumPy applications are actually not exposed to NaN problems
> because it is recommended that NaNs be avoided in computations and
> when missing or undefined values are necessary, the recommended
> solution is to use ma.array or masked array which is a drop-in
> replacement for numpy array type and carries a boolean "mask" value
> with every element.  This allows to have undefined elements is arrays
> of any type: float, integer or even boolean.  Masked values propagate
> through all computations including comparisons.

So do new masks get created when the outcome of an elementwise
operation is a NaN? Because that's the only reason why one should have
NaNs in one's data in the first place -- not to indicate missing
values!

-- 
--Guido van Rossum (python.org/~guido)