a.index(float('nan')) fails

[Steven D'Aprano]

On Thu, 25 Oct 2012 18:46:20 -0700, mamboknave wrote:

> That means, the function .index() cannot detect nan values. It happens
> on both Python 2.6 and Python 3.1
> 
> Is this a bug? Or I am not using .index() correctly?

You shouldn't be using index() or == to detect NANs. The right way to 
detect NANs is with the math.isnan() function.

The list.index method tests for the item with equality. Since NANs are 
mandated to compare unequal to anything, including themselves, index 
cannot match them.

Try this instead:

from math import isnan

def find_nan(seq):
    """Return the index of the first NAN in seq, otherwise None."""
    for i, x in enumerate(seq):
        if isnan(x):
            return i


For old versions of Python that don't provide an isnan function, you can 
do this:

def isnan(x):
    return x != x



-- 
Steven