None Python error/bug?

Question

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

>>> a = None
>>> type(a)


        

Answer 1

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

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)).

Answer 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.

Answer 4

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.