Another spin-off from the "[Python-Dev] PyObject_RichCompareBool identity shortcut" thread:
I would like to discuss another peculiarity of NaNs:
float('nan') < 0 False float('nan') > 0 False
This property in my experience causes much more trouble than nan == nan being false. The problem is that common sorting or binary search algorithms may degenerate into infinite loops in the presence of nans. This may even happen when searching for a finite value in a large array that contains a single nan. Errors like this do happen in the wild and and after chasing a bug like this programmers tend to avoid nans at all costs. Oftentimes this leads to using "magic" placeholders such as 1e300 for missing data.
Since py3k has already made None < 0 an error, it may be reasonable for float('nan') < 0 to raise an error as well (probably ValueError rather than TypeError). This will not make lists with nans sortable or searchable using binary search, but will make associated bugs easier to find.