How can I check for NaN values?

math.isnan(x)

Return True if x is a NaN (not a number), and False otherwise.

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

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

def isNaN(num):
    return num != num

numpy.isnan(number) tells you if it's NaN or not.

Here are three ways where you can test a variable is "NaN" or not.

import pandas as pd
import numpy as np
import math

# For single variable all three libraries return single boolean
x1 = float("nan")

print(f"It's pd.isna: {pd.isna(x1)}")
print(f"It's np.isnan: {np.isnan(x1)}}")
print(f"It's math.isnan: {math.isnan(x1)}}")

Output

It's pd.isna: True
It's np.isnan: True
It's math.isnan: True

here is an answer working with:

• NaN implementations respecting IEEE 754 standard
  • ie: python's NaN: float('nan'), numpy.nan...
• any other objects: string or whatever (does not raise exceptions if encountered)

A NaN implemented following the standard, is the only value for which the inequality comparison with itself should return True:

def is_nan(x):
    return (x != x)

And some examples:

import numpy as np
values = [float('nan'), np.nan, 55, "string", lambda x : x]
for value in values:
    print(f"{repr(value):<8} : {is_nan(value)}")

Output:

nan      : True
nan      : True
55       : False
'string' : False
<function <lambda> at 0x000000000927BF28> : False