[Python-ideas] [Python-Dev] Why is nan != nan?

[Mark Dickinson]

[Moving back to python-ideas]

On Sat, Mar 27, 2010 at 12:05 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> Certainly not. That defeats the whole purpose of NANs. I wish floating
> point calculations in Python would return NANs rather than raise the
> exceptions they do now.

I'd prefer the opposite:  that arithmetic operations and math
functions raise rather than produce nans;  this seems friendlier for
users who don't want to know or care about infinities and nans.  But I
agree that there are uses for nonstop mode.

<python-ideas-territory>

How about default behaviour like the above, with a "from __options__
import non-stop-arithmetic" for people who want 1./0. to give infinity
and sqrt(-1) to give nan?  In order words, default behaviour roughly
corresponds to the IEEE 754 spec with the 'invalid operation',
'overflow' and 'divide-by-zero' operations trapped (exactly as the
default context in the decimal module does currently), and the
non-stop behaviour corresponds to IEEE 754 with none of those
operations trapped and returning default values.

A more elaborate proposal would allow individual control over the
three signals above.  (Full control over the 'underflow' and 'inexact'
signals isn't really feasible given C's poor support for these
things.)

</python-ideas-territory>.

Mark