Python 3 Object-Oriented Programming - Third Edition

Singleton implementation

Python doesn't have private constructors, but for this purpose, we can use the __new__ class method to ensure that only one instance is ever created:

class OneOnly: 
    _singleton = None 
    def __new__(cls, *args, **kwargs): 
        if not cls._singleton: 
            cls._singleton = super(OneOnly, cls 
                ).__new__(cls, *args, **kwargs) 
        return cls._singleton 

When __new__ is called, it normally constructs a new instance of that class. When we override it, we first check whether our singleton instance has been created; if not, we create it using a super call. Thus, whenever we call the constructor on OneOnly, we always get the exact same instance:

    >>> o1 = OneOnly()
    >>> o2 = OneOnly()
    >>> o1 == o2
    True
    >>> o1
    <__main__.OneOnly object at 0xb71c008c> ...