[Python-Dev] PyObject_RichCompareBool identity shortcut

[Alexander Belopolsky]

On Thu, Apr 28, 2011 at 1:25 PM, Steven D'Aprano <steve at pearwood.info> wrote:
..
> But it's perfectly fine to do this:
>
>>>> sum(a)
> nan
>

This use case reminded me Kahan's

"""
Were there no way to get rid of NaNs, they would be as useless as
Indefinites on CRAYs; as soon as one were encountered, computation
would be best stopped rather than continued for an indefinite time to
an Indefinite conclusion.
""" http://www.cs.berkeley.edu/~wkahan/ieee754status/ieee754.ps

More often than not, you would want to sum non-NaN values instead.

..
> (I grant that Alexander is an exception -- I understand that he does do a
> lot of numeric work, and does come across NANs, and still doesn't like them
> one bit.)

I like NaNs for high-performance calculations, but once you wrap
floats individually in Python objects, performance is killed and you
are better off using None instead of NaN.

Python lists don't support element-wise operations and therefore there
is little gain from being able to write x + y in loops over list
elements instead of ieee_add(x, y) or add_or_none(x, y) with proper
definitions of these functions.  On the other hand, __eq__ gets
invoked implicitly in cases where you don't access to the loop.  Your
only choice is to filter your data before invoking such operations.