How can I check for NaN values?

Question

float(\'nan\') results in Nan (not a number). But how do I check for it? Should be very easy, but I cannot find it.

Answer 1

I am receiving the data from a web-service that sends NaN as a string 'Nan'. But there could be other sorts of string in my data as well, so a simple float(value) could throw an exception. I used the following variant of the accepted answer:

def isnan(value):
  try:
      import math
      return math.isnan(float(value))
  except:
      return False

Requirement:

isnan('hello') == False
isnan('NaN') == True
isnan(100) == False
isnan(float('nan')) = True