None Python error/bug?

Question

In Python you have the None singleton, which acts pretty oddly in certain circumstances:

>>> a = None
>>> type(a)
<type 'NoneType'>
>>> isinstance(a,None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

So first off, <type 'NoneType'> displays that None is not a type, but that NoneType is. Yet when you run isinstance(a,NoneType), it responds with an error: NameError: name 'NoneType' is not defined

Now, given this, if you have a function with an input default set to None, and need to check, you would do the following:

if variable is None:
    #do something
else:
    #do something

what is the reason that I cannot do the following instead:

if isinstance(variable,None): #or NoneType
    #do something
else:
    #do something

I am just looking for a detailed explanation so I can better understand this

Edit: good application

Lets say I wanted to use isinstance so that I can do something if variable is a variety of types, including None:

if isinstance(variable,(None,str,float)):
    #do something

if variable == None is anti-idiomatic Python. The standard way to do this test is by making use of the fact that None is a singleton: use if variable is None instead. — Henry Keiter
– Henry Keiter, Commented Jun 19, 2013 at 18:21

karthikr · Accepted Answer · 2013-06-19 18:20:45Z

32

You can try:

>>> variable = None
>>> isinstance(variable,type(None))
True
>>> variable = True
>>> isinstance(variable,type(None))
False

isinstance takes 2 arguments isinstance(object, classinfo) Here, by passing None you are setting classinfo to None, hence the error. You need pass in the type.

answered Jun 19, 2013 at 18:20

karthikr

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Comments

arshajii · Accepted Answer · 2013-06-19 18:36:36Z

24

None is not a type, it is the singleton instance itself - and the second argument of isinstance must be a type, class or tuple of them. Hence, you need to use NoneType from types.

from types import NoneType
print isinstance(None, NoneType)
print isinstance(None, (NoneType, str, float))

True
True

Although, I would often be inclined to replace isinstance(x, (NoneType, str, float)) with x is None or isinstance(x, (str, float)).

edited Jun 19, 2013 at 18:36

answered Jun 19, 2013 at 18:20

arshajii

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4 Comments

Ryan Saxe Over a year ago

Good answer and explanation, but I accepted karthikr because it did not require an import

arshajii Over a year ago

@RyanSaxe It's alright, I forgive you. But if you plan on using NoneType a lot I would just import it from types. It seems unnecessary to call type(None) when you can easily access that value.

Sadık Over a year ago

There is no longer a NoneType reference in types. You may use NoneType = type(None)

daparic Over a year ago

I like this answer because it normalizes my code, rather than handle the case of None differently

Ashwini Chaudhary · Accepted Answer · 2013-06-19 18:25:08Z

4

None is the just a value of types.NoneType, it's not a type.

And the error is clear enough:

TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

From the docs:

None is the sole value of types.NoneType. None is frequently used to represent the absence of a value, as when default arguments are not passed to a function.

You can use types.NoneType

>>> from types import NoneType
>>> isinstance(None, NoneType)
True

is operator also works fine:

>>> a = None
>>> a is None
True

edited Jun 19, 2013 at 18:25

answered Jun 19, 2013 at 18:20

Ashwini Chaudhary

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Comments

Karoly Horvath · Accepted Answer · 2013-06-19 18:23:37Z

3

None is a value(instance) and not a type. As the error message shows, isinstance expects the second argument to be a type.

The type of None is type(None), or Nonetype if you import it (from types import NoneType)

Note: the idiomatic way to do the test is variable is None. Short and descriptive.

answered Jun 19, 2013 at 18:23

Karoly Horvath

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