How to "test" NoneType in python?

python nonetype
CrveniZg picture CrveniZg · Apr 15, 2014 · Viewed 551.4k times · Source

I have a method that sometimes returns a NoneType value. So how can I question a variable that is a NoneType? I need to use if method, for example

if not new:
    new = '#'

I know that is the wrong way and I hope you understand what I meant.

Answer

thefourtheye picture thefourtheye · Apr 15, 2014

So how can I question a variable that is a NoneType?

Use is operator, like this

if variable is None:

Why this works?

Since None is the sole singleton object of NoneType in Python, we can use is operator to check if a variable has None in it or not.

Quoting from is docs,

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.

Since there can be only one instance of None, is would be the preferred way to check None.

Hear it from the horse's mouth

Quoting Python's Coding Style Guidelines - PEP-008 (jointly defined by Guido himself),

Comparisons to singletons like None should always be done with is or is not, never the equality operators.

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