> We cannot guarantee that NAN never equal to anything, because we can 
> create an object equal to it. For example mock.ANY

Sure. I don't expect that mock.ANY or other weird objects should obey 
the IEEE-754 rules. But we're talking numeric comparisons here, not 
arbitrary objects which may do arbitrary things in their `__eq__` 
method.

The documentation for the math module assumes we're talking about 
sensible, standard numeric values that don't play strange tricks on the 
caller. Look at the first function documented:

math.ceil(x)
Return the ceiling of x, the smallest integer greater than or equal to 
x. If x is not a float, delegates to x.__ceil__(), which should return 
an Integral value.

    class MyWeirdFloat(float):
        def __ceil__(self):
            return -1

    math.ceil(MyWeirdFloat(25.9))  # Returns -1 instead of the ceiling.

Does the documentation really need to cover every imaginable weird 
class? I don't think so. Let's keep it simple. NANs compared unequal 
with all numeric values which directly or indirectly obey IEEE-754, 
which includes floats.