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math - python nan != nan

Python 2.7.3 (default, Aug  1 2012, 05:14:39) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 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?

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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.


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