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

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
 at 0x000000000927BF28> : False