How to "test" NoneType in python?

Question

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 1

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.

Answer 2

if variable is None:
   ...

---

if variable is not None:
   ...

Answer 3

It can also be done with isinstance as per Alex Hall's answer :

>>> NoneType = type(None)
>>> x = None
>>> type(x) == NoneType
True
>>> isinstance(x, NoneType)
True

isinstance is also intuitive but there is the complication that it requires the line

NoneType = type(None)

which isn't needed for types like int and float.

Answer 4

As pointed out by Aaron Hall's comment:

Since you can't subclass NoneType and since None is a singleton, isinstance should not be used to detect None - instead you should do as the accepted answer says, and use is None or is not None.

---

Original Answer:

The simplest way however, without the extra line in addition to cardamom's answer is probably:
isinstance(x, type(None))

So how can I question a variable that is a NoneType? I need to use if method

Using isinstance() does not require an is within the if-statement:

if isinstance(x, type(None)): 
    #do stuff

---

Additional information
You can also check for multiple types in one isinstance() statement as mentioned in the documentation. Just write the types as a tuple.

isinstance(x, (type(None), bytes))

Answer 5

Not sure if this answers the question. But I know this took me a while to figure out. I was looping through a website and all of sudden the name of the authors weren't there anymore. So needed a check statement.

if type(author) == type(None):
     print("my if body")
else:
     print(" my else body")

Author can be any variable in this case, and None can be any type that you are checking for.

Answer 6

Python 2.7 :

x = None
isinstance(x, type(None))

or

isinstance(None, type(None))

==> True

Answer 7

I hope this example will be helpful for you)

print(type(None))  # NoneType

So, you can check type of the variable name

# Example
name = 12  # name = None

if type(name) is type(None):
    print("Can't find name")
else:
    print(name)

Answer 8

I mostly use the following method to check for NoneType.

if (new): # not None
  ...
else: # NoneType
  ...

Answer 9

You can test explicitly for NoneType by importing it:

>>> from types import NoneType

or, without import:

>>> NoneType = type(None)

When is this useful?

When you have a variable that's optional which can be either set to its type or to None. Now you want to validate its type using a single isinstance call:

>>> from types import NoneType
>>> foo = 'bar'
>>> assert isinstance(foo, (str, NoneType))

which is equivalent to:

>>> assert isinstance(foo, str) or isinstance(foo, NoneType)

as well as:

>>> assert isinstance(foo, str) or (foo is None)