Use math.isnan:

>>> 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

Editor’s note: The below timings are flawed, for example, they have not factored out name lookup time. See the comments.

It seems that checking if it’s equal to itself (x != x) is the fastest.

import pandas as pd 
import numpy as np 
import math 

x = float('nan')

%timeit x != x
44.8 ns ± 0.152 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

%timeit math.isnan(x)
94.2 ns ± 0.955 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

%timeit pd.isna(x)
281 ns ± 5.48 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit np.isnan(x)
1.38 µs ± 15.7 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

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

I actually just ran into this, but for me it was checking for nan, -inf, or inf. I just used

if float('-inf') < float(num) < float('inf'):

This is true for numbers, false for nan and both inf, and will raise an exception for things like strings or other types (which is probably a good thing). Also this does not require importing any libraries like math or numpy (numpy is so damn big it doubles the size of any compiled application).

math.isnan()

or compare the number to itself. NaN is always != NaN, otherwise (e.g. if it is a number) the comparison should succeed.

Well I entered this post, because i’ve had some issues with the function:

math.isnan()

There are problem when you run this code:

a = "hello"
math.isnan(a)

It raises exception.
My solution for that is to make another check:

def is_nan(x):
    return isinstance(x, float) and math.isnan(x)

Another method if you’re stuck on <2.6, you don’t have numpy, and you don’t have IEEE 754 support:

def isNaN(x):
    return str(x) == str(1e400*0)

Comparison pd.isna, math.isnan and np.isnan and their flexibility dealing with different type of objects.

The table below shows if the type of object can be checked with the given method:

+------------+-----+---------+------+--------+------+
|   Method   | NaN | numeric | None | string | list |
+------------+-----+---------+------+--------+------+
| pd.isna    | yes | yes     | yes  | yes    | yes  |
| math.isnan | yes | yes     | no   | no     | no   |
| np.isnan   | yes | yes     | no   | no     | yes  | <-- # will error on mixed type list
+------------+-----+---------+------+--------+------+

pd.isna

The most flexible method to check for different types of missing values.

None of the answers cover the flexibility of pd.isna. While math.isnan and np.isnan will return True for NaN values, you cannot check for different type of objects like None or strings. Both methods will return an error, so checking a list with mixed types will be cumbersom. This while pd.isna is flexible and will return the correct boolean for different kind of types:

In [1]: import pandas as pd

In [2]: import numpy as np

In [3]: missing_values = [3, None, np.NaN, pd.NA, pd.NaT, '10']

In [4]: pd.isna(missing_values)
Out[4]: array([False,  True,  True,  True,  True, False])

With python < 2.6 I ended up with

def isNaN(x):
    return str(float(x)).lower() == 'nan'

This works for me with python 2.5.1 on a Solaris 5.9 box and with python 2.6.5 on Ubuntu 10

I am receiving the data from a web-service that sends NaN as a string 'Nan'. But there could be other sorts of string in my data as well, so a simple float(value) could throw an exception. I used the following variant of the accepted answer:

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

Requirement:

isnan('hello') == False
isnan('NaN') == True
isnan(100) == False
isnan(float('nan')) = True

All the methods to tell if the variable is NaN or None:

None type

In [1]: from numpy import math

In [2]: a = None
In [3]: not a
Out[3]: True

In [4]: len(a or ()) == 0
Out[4]: True

In [5]: a == None
Out[5]: True

In [6]: a is None
Out[6]: True

In [7]: a != a
Out[7]: False

In [9]: math.isnan(a)
Traceback (most recent call last):
  File "<ipython-input-9-6d4d8c26d370>", line 1, in <module>
    math.isnan(a)
TypeError: a float is required

In [10]: len(a) == 0
Traceback (most recent call last):
  File "<ipython-input-10-65b72372873e>", line 1, in <module>
    len(a) == 0
TypeError: object of type 'NoneType' has no len()

NaN type

In [11]: b = float('nan')
In [12]: b
Out[12]: nan

In [13]: not b
Out[13]: False

In [14]: b != b
Out[14]: True

In [15]: math.isnan(b)
Out[15]: True

In Python 3.6 checking on a string value x math.isnan(x) and np.isnan(x) raises an error.
So I can’t check if the given value is NaN or not if I don’t know beforehand it’s a number.
The following seems to solve this issue

if str(x)=='nan' and type(x)!='str':
    print ('NaN')
else:
    print ('non NaN')

How to remove NaN (float) item(s) from a list of mixed data types

If you have mixed types in an iterable, here is a solution that does not use numpy:

from math import isnan

Z = ['a','b', float('NaN'), 'd', float('1.1024')]

[x for x in Z if not (
                      type(x) == float # let's drop all float values…
                      and isnan(x) # … but only if they are nan
                      )]

['a', 'b', 'd', 1.1024]

Short-circuit evaluation means that isnan will not be called on values that are not of type ‘float’, as False and (…) quickly evaluates to False without having to evaluate the right-hand side.

For nan of type float

>>> import pandas as pd
>>> value = float(nan)
>>> type(value)
>>> <class 'float'>
>>> pd.isnull(value)
True
>>>
>>> value = 'nan'
>>> type(value)
>>> <class 'str'>
>>> pd.isnull(value)
False

If you want to check for values that are not NaN, then negate whatever is used to flag NaNs; pandas has its own dedicated function for flagging non-NaN values.

lst = [1, 2, float('nan')]

m1 = [e == e for e in lst]              # [True, True, False]

m2 = [not math.isnan(e) for e in lst]   # [True, True, False]

m3 = ~np.isnan(lst)                     # array([ True,  True, False])

m4 = pd.notna(lst)                      # array([ True,  True, False])

This is especially useful if you want to filter values that are not NaN. For ndarray/Series objects, == is vectorized, so it can be used as well.

s = pd.Series(lst)
arr = np.array(lst)

x = s[s.notna()]
y = s[s==s]                             # `==` is vectorized
z = arr[~np.isnan(arr)]                 # array([1., 2.])

assert (x == y).all() and (x == z).all()

To filter out both empty strings (''), None and NaN values in the ‘num_specimen_seen’ column, we can use the pd.notna() function from pandas.

import pandas as pd
import numpy as np

df = pd.DataFrame({
    'num_specimen_seen': [10, 2, 1, '', 34, 'aw', np.NaN, 5, '43', np.nan, 'ed', None, '']
})

for idx, row in df.iterrows():
    if pd.notna(row['num_specimen_seen']) and row['num_specimen_seen'] != '':
        print(idx, row['num_specimen_seen'])

This code will skip both NaN and empty strings in the ‘num_specimen_seen’ column when iterating over the DataFrame.

for strings in panda take pd.isnull:

if not pd.isnull(atext):
  for word in nltk.word_tokenize(atext):

the function as feature extraction for NLTK

def act_features(atext):
features = {}
if not pd.isnull(atext):
  for word in nltk.word_tokenize(atext):
    if word not in default_stopwords:
      features['cont({})'.format(word.lower())]=True
return features