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

In Python 3.6 checking on a string value x math.isnan(x) and np.isnan(x) raises an error. So I can't check if the given value is NaN or not if I don't know beforehand it's a number. The following seems to solve this issue

if str(x)=='nan' and type(x)!='str':
    print ('NaN')
else:
    print ('non NaN')

Answer 2

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

def isNaN(num):
    return num != num

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

for strings in panda take pd.isnull:

if not pd.isnull(atext):
  for word in nltk.word_tokenize(atext):

the function as feature extraction for NLTK

def act_features(atext):
features = {}
if not pd.isnull(atext):
  for word in nltk.word_tokenize(atext):
    if word not in default_stopwords:
      features['cont({})'.format(word.lower())]=True
return features

Answer 5

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