Python Class Inherited Singleton Inits Instance On Every Call

I'm trying to implement class inherited singleton as described here (Method 2). Going over the question and the extensive chosen answer I tried to implement the following: class Si

Solution 1:

In modern Python this could be done by writting a __init_subclass__ method that could decorate __init__ to make it check cls._instance before running.

As it is, (i.e. you need it for Python 2 as well) - I think the simpler thing is for __new__ to patch __init__ with a NOP method if the instance already exists:

_nop_init = lambda self, *args, **kw: NoneclassSingleton(object):
    _instance = Nonedef__new__(cls, *args, **kwargs):
        ifnotisinstance(cls._instance, cls):
            cls._instance = object.__new__(cls, *args, **kwargs)
            cls._instance._initialized = False# Python 2 have to check in the cls.__dict__ - Py3 could check the attribute directly:elif cls.__dict__.get("__init__", None) isnot _nop_init:
            cls.__init__ = _nop_init
        return cls._instance

classA(Singleton):
    def__init__(self):
        print"Init is called"classB(Singleton):
    def__init__(self):
        print"Init is called"

extra info The language mechanism to call __init__ is built-in the __call__ method of type - the metaclass for all classes in Python. It will call the __new__ and __init__ method of a target class when instantiating it, and thus, with a metaclass, it is easy to control these calls from a custom metaclass. It is also interesting to note that when __new__ does not return an instance of the target class, __init__ is not called. (In this singleton case, the singleton is an instance of te class, and thus it is called).

real world example: last time I coded a singleton, I opted for avoiding re-run __init__ in its own code - sine it is the only singleton in the project, tere was no need for generic code for that in a __new__: https://github.com/jsbueno/pythonchain/blob/1f9208dc8bd2741a574adc1bf745d218e4314e4a/pythonchain/block.py#L276