[issue12211] Better document math.copysign behavior.

[Mark Dickinson]

Mark Dickinson added the comment:

The paragraph about NaNs isn't correct.  For example:

>>> from math import copysign
>>> copysign(1.0, float('nan'))
1.0
>>> copysign(1.0, -float('nan'))
-1.0

Though it doesn't really make sense to talk about 'positive' or 'negative' NaNs, a NaN still has a sign bit, and that sign bit is used in the copysign operation.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue12211>
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