705

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.

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3
  • I think this was answered here and apparently somewhere before Commented Apr 15, 2014 at 14:21
  • If None is the only value your method returns for which bool(returnValue) equals False, then if not new: ought to work fine. This occurs sometimes in the built-in libs - for example, re.match returns either None or a truthy match object. Commented Apr 15, 2014 at 14:21
  • Also see my answer about null and None in python here. Commented May 8, 2018 at 9:25

9 Answers 9

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1030

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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9 Comments

So what's the difference between == None and is None?
@NoName With respect to None, they both will yield similar result.
this doesn't work if comparing whether a pandas DataFrame exists -- for that, I use type(df) is type(None) to avoid: The truth value of a DataFrame is ambiguous
@NoName See Python None comparison: should I use “is” or ==?
isinstance(instance, type(None)) should be added to make this a complete answer. There are edge cases. For example, if you are building a dynamic tuple of types for validation and the validator is using isinstance (which takes a tuple).
|
142 
if variable is None:
   ...

if variable is not None:
   ...
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75

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.

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5 Comments

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.
There are edge cases. For example, if you are building a dynamic list of types for validation and the validator is using isinstance (which takes a list).
Correction: tuple, not list
Also, assert isinstance(x, str) or x is None will not result in x being inferred as str | None by Pyright, but assert isinstance(x, (str, type(None))) will.
Given that current doc of isinstance says nothing about this case I suppose this is valid Python code. However if you use mypy be aware of this currently-open issue.
38

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))
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3 Comments

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.
@AaronHall Why isinstance should not be used ? I understand that is should be preferred, but there are some cases where the isinstance form feels more natural (like checking for multiple types at once isinstance(x, (str, bool, int, type(None)))). Is it just a personal preference or is there caveat that I'm unaware of ?
@Conchylicultor downsides to your suggestion: 1. global look-up for type 2. then calling it 3. then looking up the type of None - when None is both a singleton and a keyword. Another downside: 4. this is very non-standard and will raise eyebrows when people are looking at your code. x is None is a more optimized check. I would suggest x is None or isinstance(x, (str, bool, int)) - but I would also suggest you think more about what you're doing when you're doing that kind of type checking for types that don't have a lot in common...
19

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.

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3 Comments

Since None is a singleton, type should not be used to detect None - instead you should do as the accepted answer says, and use is None or is not None.
WTH is my else body?!
@jtlz2 sorry I should have clarified. My else body is whatever code you want to happen if the else statement executes. Its not a protected python wording if you're wondering
5

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)
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2

Python 2.7 :

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

or

isinstance(None, type(None))

==> True

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2 Comments

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.
How this is copied to every answer, nice --
0

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)
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1 Comment

Because None is a singleton and None is the only instance of NoneType, your example is more complicated than necessary. Just check your variable for None: if name is None: ...
0

I mostly use the following method to check for NoneType.

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