pythonic design patterns

singleton.py

class Example(object):
    """A singleton not thread safe
    """

    _instance = None

    @classmethod
    def instance(cls):
        if cls._instance is None:
            cls._instance = cls.__new__(cls)
            cls._instance._attrs = []
        return cls._instance
      
    def __init__(self):
       raise RuntimeError('This is a singleton, call instance() instead.'