If I'm not mistaken, signaling NaNs are only created when the user
explicitly initializes a variable. I see this as direct request to
raise an exception whenever the variable is accessed in a way that
changes the outcome of the program:

This is the example I gave:

http://mail.python.org/pipermail/python-dev/2009-November/093952.html


Now, ideally one would still be allowed to store signaling NaNs in
a dictionary and have them raise at the _exact_ location where they
are used in a mathematical operation or influence control flow.

But since that's not possible, I prefer things as they are.


+1 for adding a comment to the hash function.