Use math.isnan:

>>> import math
>>> x = float('nan')
>>> math.isnan(x)
True

A float is a 64-bit* IEEE-754 Floating-Point Number.

Most values that can be represented by floats, we can write as literals (e.g. 1.5 or 52.0). However, the values for Infinity (inf) and "Not-a-Number" (NaN), which the IEEE-754 standard requires, can only be reliably assigned by converting those strings into floats. That's what float('nan') does - it creates a float whose value is NaN.

The article you read is misleading. According to the IEEE-754 standard, the value NaN works differently from any other float value - it will always return false when used in any comparison with another number, unless that comparison is !=, in which case it will always return true. This is even the case when comparing it to itself:

Use with AI>>> var = float('nan')
>>> var != var
True

as such, using NaN in comparisons is not very useful.

Instead, to check if a variable contains the value NaN, do

Use with AIimport math
...
if math.isnan(some_variable):
    ...

or just

Use with AIif some_variable == some_variable:
    ...

since that will always be true if some_variable is not NaN, and will always be false if it is NaN.

The article is probably guiding you to check whether the values in a column of the dataframe are NaN and use different behavior accordingly.

*in python, at least - in most languages float is 32 bits and double is the 64-bit equivalent

Yes -- use math.nan.

>>> from math import nan
>>> print(nan)
nan
>>> print(nan + 2)
nan
>>> nan == nan
False
>>> import math
>>> math.isnan(nan)
True

Before Python 3.5, one could use float("nan") (case insensitive).

Note that checking to see if two things that are NaN are equal to one another will always return false. This is in part because two things that are "not a number" cannot (strictly speaking) be said to be equal to one another -- see What is the rationale for all comparisons returning false for IEEE754 NaN values? for more details and information.

Instead, use math.isnan(...) if you need to determine if a value is NaN or not.

Furthermore, the exact semantics of the == operation on NaN value may cause subtle issues when trying to store NaN inside container types such as list or dict (or when using custom container types). See Checking for NaN presence in a container for more details.

You can also construct NaN numbers using Python's decimal module:

>>> from decimal import Decimal
>>> b = Decimal('nan')
>>> print(b)
NaN
>>> print(repr(b))
Decimal('NaN')
>>>
>>> Decimal(float('nan'))
Decimal('NaN')
>>> 
>>> import math
>>> math.isnan(b)
True

math.isnan(...) will also work with Decimal objects.

However, you cannot construct NaN numbers in Python's fractions module:

>>> from fractions import Fraction
>>> Fraction('nan')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python35\lib\fractions.py", line 146, in __new__
    numerator)
ValueError: Invalid literal for Fraction: 'nan'
>>>
>>> Fraction(float('nan'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python35\lib\fractions.py", line 130, in __new__
    value = Fraction.from_float(numerator)
  File "C:\Python35\lib\fractions.py", line 214, in from_float
    raise ValueError("Cannot convert %r to %s." % (f, cls.__name__))
ValueError: Cannot convert nan to Fraction.

Incidentally, you can also do float('Inf'), Decimal('Inf'), or math.inf (3.5+) to assign infinite numbers. (And also see math.isinf(...))

However doing Fraction('Inf') or Fraction(float('inf')) isn't permitted and will throw an exception, just like NaN.

If you want a quick and easy way to check if a number is neither NaN nor infinite, you can use math.isfinite(...) as of Python 3.2+.

If you want to do similar checks with complex numbers, the cmath module contains a similar set of functions and constants as the math module:

• cmath.isnan(...)
• cmath.isinf(...)
• cmath.isfinite(...) (Python 3.2+)
• cmath.nan (Python 3.6+; equivalent to complex(float('nan'), 0.0))
• cmath.nanj (Python 3.6+; equivalent to complex(0.0, float('nan')))
• cmath.inf (Python 3.6+; equivalent to complex(float('inf'), 0.0))
• cmath.infj (Python 3.6+; equivalent to complex(0.0, float('inf')))

Cast from string using float():

>>> float('NaN')
nan
>>> float('Inf')
inf
>>> -float('Inf')
-inf
>>> float('Inf') == float('Inf')
True
>>> float('Inf') == 1
False

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.