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Fri Mar 19 10:29:40 EST 2004 · http://mail.python.org/pipermail/python-dev/2004-March/043287.html

Michael Hudson wrote:
>  >>> float('nan')
>  nan
>  >>> _ == _
>  False

This means that 'nan' is no longer a well-behaved dictionary key:

     >>> x = {float('nan'):0}
     >>> x[float('nan')] = 1
     >>> print x
     {nan: 0, nan: 1}

Even worse, we get different behavior if we use the "same copy" of nan:

     >>> nan = float('nan')
     >>> x = {nan:0}
     >>> x[nan] = 1
     >>> print x
     {nan: 1}

If we *really* want nan==nan to be false, then it seems like we have to 
say that nan is unhashable.  I'm also disturbed by the fact that cmp() 
has something different to say about their equality:

     >>> cmp(float('nan'), float('nan'))
     0

-Edward

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