python

How to inherit from Python None

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

I tried this:

class InvalidKeyNone(None):
    pass

but that gives me:

TypeError: Error when calling the metaclass bases
    cannot create 'NoneType' instances

What would be the correct solution that gives me a type that behaves exactly like None but which I can type test?

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

I want to do this because I am creating a selection scheme on Python datastructures:

bar = select(".foo.bar.[1].x", {"foo":{"bar":[{"x":1}, {"x":2}], "baz":3})
print(bar)
>> 2

And I want to be able to distinguish whether I get a None because the selected value is None or because the key was not found.

However, it must return a (ducktyped) None that behaves exactly like a None. No exceptions or custom type returning here.

I really want the default behavior to have it return a None when the key is not present.

Solution

  • None is a constant, the sole value of NoneType (which is in types in v2.7 and v3.10+)

    Anyway, when you try to inherit from NoneType:

    from types import NoneType  # Python 2.7 or 3.10+

# NoneType = type(None)  # Python 3.0-3.9

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

    TypeError: type 'NoneType' is not an acceptable base type

    in short, you cannot inherit from NoneType.