How to inherit from Python None

Question

I would like to create a class that inherites from None.

Tried this:

class InvalidKeyNone(None):
    pass

but that giv

Answer 1

There's no way to do it, at least definitely not until you do some unreadable black magic.

You should probably use exceptions.

Answer 2

None is a constant, the sole value of types.NoneType (for v2.7, for v3.x)

Anyway, when you try to inherit from types.NoneType

from types import NoneType

class InvalidKeyNone(NoneType):
    pass

foo = InvalidKeyNone()
print(type(foo))

you'll get this error

Python 2

TypeError: Error when calling the metaclass bases type 'NoneType' is not an acceptable base type

Python 3

ImportError: cannot import name 'NoneType'

in short, you cannot inherit from NoneType

Anyway, why would want a class to inherit NoneType?

Answer 3

Subclassing None does not make sense, since it is a singleton and There Can Be Only One. You say you want a class with the same behaviour, but None does not have any behaviour!

If what you really want is a unique placeholder that you can return from a function to indicate a special case then simplest way to do this is to create a unique instance of object:

InvalidKey = object()

result = doSomething()
if result is InvalidKey:
    ...