Implementing Singleton pattern results in TypeError: unbound method foobar() must be called with Singleton instance as first argument

I'm trying to implement the Singleton pattern in Python (2.7).

I've read severel posts (1, 2, 3, 4) about the implementation and I want to code my own version. (A version which I understand. I'm new to Python.)

So I'm creating the singleton with a method that will create my single object itself that will be returned on every Singleton.Instance() call.

But the error message is always the same:

Traceback (most recent call last):
  File "./test4.py", line 24, in <module>
    print id(s.Instance())
  File "./test4.py", line 15, in Instance
    Singleton._instance = Singleton._creator();
TypeError: unbound method foobar() must be called with Singleton instance as first argument (got nothing instead)

Here I roll:

class Singleton(object):

    _creator = None
    _instance = None

    def __init__(self, creator):
        if Singleton._creator is None and creator is not None:
            Singleton._creator = creator

    def Instance(self):

        if Singleton._instance is not None:
            return Singleton._instance

        Singleton._instance = Singleton._creator();

        return Singleton._instance;

def foobar():
    return "foobar"

s = Singleton( foobar )

print id(s.Instance())

Why is that? To be more specific: How do I call in a method a def stored in a class variable in Python?