How to check for float('nan') in Python?

Question

In some data I am processing I am encountering data of the type float, which are filled with 'nan', i.e. float('nan').

However checking for it does not work as expected:

float('nan') == float('nan')
>> False

You can check it with math.isnan, but as my data also contains strings (For example: 'nan', but also other user input), it is not that convenient:

import math
math.isnan(float('nan'))
>> True
math.isnan('nan')
>> TypeError: must be real number, not str

In the ideal world I would like to check if a value is in a list of all possible NaN values, for example:

import numpy as np
if x in ['nan', np.nan, ... ]:
    # Do something
    pass

Now the question: How can I still use this approach but also check for float('nan') values? And why equals float('nan') == float('nan') False

Answer 1

Why not just wrap whatever you pass to math.isnan with another float conversion? If it was already a float (i.e. float('nan')) you just made a "redundant" call:

import math

def is_nan(value):
    return math.isnan(float(value))

And this seems to give your expected results:

>>> is_nan(float('nan'))
True
>>> is_nan('nan')
True
>>> is_nan(np.nan)
True
>>> is_nan(5)
False
>>> is_nan('5')
False

---

This will still raise a ValueError for non-numeric (except 'nan') strings. If that's a problem, you can wrap with try/except. As long as the float conversion worked, there is no reason for isnan to fail. So we are basically catching non-numeric strings that my fail the float conversion:

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

Any non-numeric string is surely not a NaN value so return False.