Terry Reedy wrote:
> On 4/27/2011 2:41 PM, Glenn Linderman wrote:
>
>> One issue that I don't fully understand: I know there is only one
>> instance of None in Python, but I'm not sure where to discover whether
>> there is only a single, or whether there can be multiple, instances of
>> NaN or Inf.
>
> I am sure there are multiple instances with just one bit pattern, the
> same as other floats. Otherwise, float('nan') would have to either
> randomly or systematically choose from among the possibilities. Ugh.
I think Glenn is asking whether NANs are singletons. They're not:
>>> x = float('nan')
>>> y = float('nan')
>>> x is y
False
>>> [x] == [y]
False
> There are functions in the math module that pull apart (and put
> together) floats.
>
>> The IEEE 754 spec is clear that there are multiple bit
>> sequences that can be used to represent these,
>
> Anyone actually interested in those should use C or possibly the math
> module float assembly function.
I'd like to point out that way back in the 1980s, Apple's Hypercard
allowed users to construct, and compare, distinct NANs without needing
to use C or check bit patterns. I think it is painful and ironic that a
development system aimed at non-programmers released by a company
notorious for "dumbing down" interfaces over 20 years ago had better and
simpler support for NANs than we have now.
--
Steven
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