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

User · Answer

if variable is None             if variable is not None

User · Answer

It can also be done with isinstance as per Alex Hall s answer      gt  gt  gt  NoneType   type None   gt  gt  gt  x   None  gt  gt  gt  type x     NoneType True  gt  gt  gt  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

User · Answer

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

User · Answer

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  quot Can t find name quot   else      print name

User · Answer

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

User · Answer

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        my if body else      my else body   Author can be any variable in this case  and None can be any type that you are checking for

User · Answer

Python 2 7    x   None isinstance x  type None     or  isinstance None  type None         True

[python] How to "test" NoneType in python?

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