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

[Ethan Furman]

Steven D'Aprano wrote:
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.

This is incorrect.  .index() uses identity first, then equality, and 
will match the same NaN in a list.  The OP's problem was in using a 
different NaN.

Having said that, your find_nan() solution is probably the one to use 
anyway.

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

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