# Assigning a variable NaN in python without numpy

Most languages have a NaN constant you can use to assign a variable the value NaN. Can python do this without using numpy?

Yes -- use `float('nan')`

. As of Python 3.5, you can also use `math.nan`

.

```
>>> a = float('nan')
>>> print(a)
nan
>>> print(a + 2)
nan
>>> a == a
False
>>> import math
>>> math.isnan(a)
True
>>> # Python 3.5+
>>> math.isnan(math.nan)
True
```

The `float(...)`

function is case-insensitive -- doing `float('NAN')`

or `float('naN')`

or similar things will also work.

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'))`

)

From: stackoverflow.com/q/19374254

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