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Sorry, but all behavior in the presence of NaNs and infinities 
and signed zeroes is a platform- and release- dependent 
accident.  Even that float("inf") didn't raise an exception for 
you is an accident (e.g., in Python 2.2.2 on Windows, it 
does raise an exception).

I've added this to PEP 42's "non-accidental 754 support" 
feature request; you may also be interested in PEP 754 
(support for 754 special values).  Python has no 754 story 
now.  If you want it to have one, consider volunteering work 
toward that end.