Mark Dickinson added the comment:
About the Python 2.7 behaviour:
>>> from math import copysign
>>> x = float("-nan")
>>> copysign(1.0, x)
1.0
I'd be interested to know what `struct.pack('<d', x)` shows in this case. I'd
expect it to be '\x00\x00\x00\x00\x00\x00\xf8\xff', meaning that the `float`
conversion has produced a value with its sign bit set, as expected, but
`copysign` has failed to transfer that sign bit. That failure is somewhat
expected: older versions of MSVC don't provide copysign, so it has to be
emulated, and the emulation doesn't take the sign of NaNs into account.
(Getting the sign of a NaN is awkward to do without a native copysign
function.) It works as expected on OS X and Linux.
So that's a separate issue: copysign on Windows / Python 2.7 doesn't correctly
handle the sign bit of a NaN. I agree that that's less than ideal, but I'm not
sure whether it's worth fixing for Python 2.7.
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<http://bugs.python.org/issue26785>
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