math - python nan != nan

Question

Python 2.7.3 (default, Aug  1 2012, 05:14:39) 
[GCC 4.6.3] on linux2
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>>> x = float('nan')
>>> id(x) == id(x)
True
>>> x == x
False

I'm interested in how nan != nan in python. And just to clarify, I know nan is supposed to behave like that by definition, I'm asking about how not about why. Where is that implemented? Is there any other object which behaves like that?

Answer 1

Not A Number (NaN) is unequal with anything. To detect it, use math.isnan. And an object like this is quite easy to define:

class A(object):
    def __eq__(self, other):
        return False

    def __ne__(self, other):
        return True

The reason why this is is quite simple. CPython follows the IEEE 754 standard for floating point math. NaN is a floating point value for which IEEE 754 dictates that it is not equal to any other floating point value.