Serhiy Storchaka added the comment:
There are several different NaNs.
>>> x = struct.unpack('d', b'\x00\x00\x00\x00\x00\x00\xf8\x7f')[0]
>>> x
nan
>>> x == x
False
>>> struct.pack('d', x)
b'\x00\x00\x00\x00\x00\x00\xf8\x7f'
>>> x = struct.unpack('d', b'\x00\x00\x00\x00\x00\x00\xf9\x7f')[0]
>>> x
nan
>>> x == x
False
>>> struct.pack('d', x)
b'\x00\x00\x00\x00\x00\x00\xf9\x7f'
Interesting, but 0*inf and inf-inf return values with the same representation
as float('-nan'), not float('nan').
>>> inf = float("inf")
>>> struct.pack('d', 0*inf)
b'\x00\x00\x00\x00\x00\x00\xf8\xff'
>>> struct.pack('d', inf-inf)
b'\x00\x00\x00\x00\x00\x00\xf8\xff'
>>> struct.pack('d', float('nan'))
b'\x00\x00\x00\x00\x00\x00\xf8\x7f'
>>> struct.pack('d', float('-nan'))
b'\x00\x00\x00\x00\x00\x00\xf8\xff'
----------
nosy: +serhiy.storchaka
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<http://bugs.python.org/issue23185>
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