repr of -nan value should contain the sign so the round-trip could be assured. NaN value sign (bit) could be seen as not relevant or even uninterpretable information, but it is actually used in real-life situations, the fact substantiated by section 6.3 of IEEE-754 2008 standard.

>>> from math import copysign
>>> x = float("-nan")
>>> copysign(1.0, x)
-1.0

This is correct. Also proves the value contains the sign information.

>>> repr(x)
nan

Not correct. Should be '-nan'.