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 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)).