float("nan") in set or as key

[Grant Edwards]

On 2011-06-01, Chris Angelico <rosuav at gmail.com> wrote:
> On Wed, Jun 1, 2011 at 12:59 PM, Carl Banks <pavlovevidence at gmail.com> wrote:
>> On Sunday, May 29, 2011 7:53:59 PM UTC-7, Chris Angelico wrote:
>>> Okay, here's a question. The Python 'float' value - is it meant to be
>>> "a Python representation of an IEEE double-precision floating point
>>> value", or "a Python representation of a real number"?
>>
>> The former. ?Unlike the case with integers, there is no way that I know of to represent an abstract real number on a digital computer.
>
> This seems peculiar. Normally Python seeks to define its data types
> in the abstract and then leave the concrete up to the various
> implementations - note,

But, "real numbers" and "IEEE float" are so different that I don't
think that it would be a wise decision for people to pretend they're
working with real numbers when in fact they are working with IEEE
floats.

> for instance, how Python 3 has dispensed with 'int' vs 'long' and
> just made a single 'int' type that can hold any integer.

Those concepts are much closer than "real numbers" and "IEEE floats".

> Does this mean that an implementation of Python on hardware that has
> some other type of floating point must simulate IEEE double-precision
> in all its nuances?

I certainly hope so.  I depend on things like propogation of
non-signalling nans, the behavior of infinities, etc.

> I'm glad I don't often need floating point numbers. They can be so
> annoying!

They can be -- especially if one pretends one is working with real
numbers instead of fixed-length binary floating point numbers.  Like
any tool, floating point has to be used properly. Screwdrivers make
very annoying hammers.

-- 
Grant Edwards               grant.b.edwards        Yow! How's it going in
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                              gmail.com