On Fri, Nov 09, 2018 at 01:17:09PM -0800, Chris Barker via Python-Dev wrote:
> works for me, too:
>
> In [9]: x = cast_int2float(0x7ff8000000000001)
> In [10]: hex(cast_float2int(x))
> Out[10]: '0x7ff8000000000001'
>
> In [11]: x = cast_int2float(0x7ff0000000000001)
> In [12]: hex(cast_float2int(x))
> Out[12]: '0x7ff0000000000001'
Fascinating. I borrowed a Debian system and tried it on there, and got
the same results as you. So I wonder whether it is something unusual
about my Red Hat system that it prevents the formation of signalling
NANs?
However, I don't think that explains why the float constructor doesn't
allow Decimal('snan') to be converted to a float.
> I suspect it depends on the compiler's math library
Unfortunately that's probably true.
> But neither is raising an exception:
[...]
> When should it?
I think that, by default, any arithmetic operation, comparison or math
library function call ought to raise if given a snan, if the underlying
math library supports IEEE-754 signals. Which I imagine these days
nearly all should do.
So any of these should raise:
snan + 1
math.sin(snan)
snan == 0
--
Steve
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