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', ie 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

It's very pedestrian, and a bit ugly, but why not just do the following?

import math
import numpy as np

if math.isnan(x) or x in ['nan', np.nan, ... ]:
    # Do something
    pass

I want to recommend a Yoda expression but haven't quite worked it out yet.

If you want to sweep everything under the carpet put it in a lambda or function.

Following on from https://stackoverflow.com/a/64090763/1021819 , you can try to get the iterator to evaluate any in a lazy fashion. The problem then is that if none of the first conditions evaluates to True then the math.isnan() call is executed and can still throw the TypeError . If you evaluate lazily you can guard the math.isnan() call with a type check against str :

fn_list_to_check=[
    lambda x: x in ['nan', np.nan, ... ],
    lambda x: not isinstance(x, str),
    lambda x: math.isnan(x)
    ]

if any(f(x) for f in fn_list_to_check):
    # Do something
    pass

Note the absence of square list brackets in the any ie any() not any([]) (who knew?).

I think it's quite brilliant but equally as ugly - choose your poison.

For the second part of the question (why float('nan') != float('nan') ), see

What is the rationale for all comparisons returning false for IEEE754 NaN values?

Answer 2

Why not just wrap whatever you pass to math.isnan with another float conversion? If it was already a float (ie 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

Answer 3

You can check for NaN value like this,

def isNaN(num):
    return num!= num

NaN represents as 'Not A Number'