[Python-ideas] math.inf and math.nan constants

Andrew Barnert abarnert at yahoo.com
Sun Jan 11 05:02:15 CET 2015


On Jan 10, 2015, at 15:22, Alexander Heger <python at 2sn.net> wrote:

>> According to the 2008 standard, the quiet bit is part of the bits that distinguish nan from inf, and that bit is set for quiet; therefore, a qnan with payload 0 exists, and is not an inf.
> 
> yes, the default payload is 0 for the quiet NaNs. I suppose the
> routine can throw an exception if one tries to make a signaling NaN
> with payload 0.  Maybe setting default payload to 1 was not a good
> suggestion.  On could set it to 1 if someone creates a signaling NaN
> w/o specifying payload (instead)...but, then, if someone goes all the
> way specifying a signaling NaN, they should know what they are doing

Well, my code wasn't intended as a stdlib proposal; if you copy it into your app, hopefully it's because you have a need for it and therefore know what you're doing. :)

> I also have not been able to find any documentation on specific NaN
> payload values/constants online so far. ... I find it somewhat a
> surprise this is not well documented ...

I don't know if anyone ever tried to standardize a set of meanings for the payload (I can imagine Apple pushing for SANE's codes, or NASA/JPL trying to get vendors to agree on something), but if so, they apparently never succeeded, because the latest standard (2008) leaves all the bits to user/library code and says the implementation should just preserve and ignore them. There seem to be a lot more blogs and such by committee members and participants nowadays, so there's a good chance that you can find some of the history if you're interested.

If you're just looking for something to use as a guide for how to assign payloads in your own project, the list of SANE codes earlier in this thread is probably not a bad place to start. (And you've still got 44 bits left if you want to track the module or something else along with the reason code.)


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