isnan() to check number is Nan , Not a number or not in Python

isnan(number) returns Boolean , True if the input number is NaN ( Not a Number ), False otherwise.

isnan(n)

Example

import math
print(math.isnan(2))     # False
print(math.isnan(-2))    # False
print(math.isnan(0.0))   # False
print(math.isnan(0/1))   # False
print(math.isnan(0.0/1)) # False

Note : 0.0 is considered finite.

import math
print(math.isnan(float('nan')))  # True
print(math.isinf(float('inf')))  # True
print(math.isinf(float('-inf'))) # True

import math
result = math.isnan(float('inf') / float('inf'))
print(result)  # Output: True

Checking with Lists:

import math
numbers = [float('nan'), 1.0, -3.5, float('inf')]
for num in numbers:
    print(math.isnan(num))

Output

True
False
False
False

Practical Differences:
NaN: Not a real number; cannot be used in calculations (results in NaN propagation).
inf and -inf: Valid but represent extreme values; still participate in arithmetic (e.g., inf + 1 = inf).

import math
numbers = [float('nan'), float('inf'), -float('inf'), 3.5]

for num in numbers:
    if math.isnan(num):
        print(f"{num} is NaN")
    elif math.isinf(num):
        print(f"{num} is Infinity")
    else:
        print(f"{num} is a valid number")

Use Case: Handling User Input: In data processing, isnan() can be used to detect invalid numbers from user inputs or sensor readings, preventing crashes.

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