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

Answer 1

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.

Answer 2

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)).