How to check for NaN in python?

Question

float('nan') results in a thingy simply called nan. But how do I check for it? Should be very easy, but i cannot find it.

Answer 1

The usual way to test for a NaN is to see if it's equal to itself:

def isNaN(num):
    return num != num

Answer 2

math.isnan()

Checks if the float x is a NaN (not a number). NaNs are part of the IEEE 754 standards. Operation like but not limited to inf * 0, inf / inf or any operation involving a NaN, e.g. nan * 1, return a NaN.

New in version 2.6.

>>> import math
>>> x=float('nan')
>>> math.isnan(x)
True
>>>

Answer 3

math.isnan()

or compare the number to itself. NaN is always != NaN, otherwise (e.g. if it is a number) the comparison should succeed.

Answer 4

numpy.isnan(float) tells you if it's NaN or not in Python 2.5

Answer 5

Another method if you're stuck on <2.6, you don't have numpy, and you don't have IEEE 754 support:

def isNaN(x):
    return str(x) == str(1e400*0)

Answer 6

With python < 2.6 I ended up with

def isNaN(x):
    return str(float(x)).lower() == 'nan'

This works for me with python 2.5.1 on a Solaris 5.9 box and with python 2.6.5 on Ubuntu 10