`NaN`, standing for **not a number**, is a numeric **data type** used to represent any value that is undefined or unpresentable. 

For example, 0/0 is undefined as a real number and is, therefore, represented by NaN. **The square root of a negative number** is an imaginary number that cannot be represented as a real number, so, it is represented by NaN.

NaN is also assigned to variables, in a computation, that do not have values and have yet to be computed.

> Note: NaN is **not** the same as `infinity` in Python

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## Assigning a NaN

We can create a NaN value in python using `float`, as shown below: 

>Note: Note that the "NaN" passed to the float is **not** case sensitive. All of the 4 variables come out as nan.

n1 = float("nan")
n2 = float("Nan")
n3 = float("NaN")
n4 = float("NAN")
print n1, n2, n3, n4

> Note: You can also use Python's `Decimal` library instead of floats. For example, you can do Decimal("Nan") instead of float("Nan").

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NaN is also part of the math module in **Python 3.5** and onwards. This can used as shown below:

import math

n1 = math.nan
print(n1)
print(math.isnan(n1))

`math.isnan` is used to check whether a certain variable is NaN or not. We **cannot** use the regular comparison operator, `==`, to check for NaN. NaN is **not equal** to anything (**not even itself!**).

How to assign NaN to a variable in Python

Assigning a NaN