Understanding metaclass and Singleton in Python

metaclass.png

REMARK.md

How to use metaclass to implement Singleton in Python

First, you need to know that instance of metaclass is class,
just like : an instance of class is object, they have similar idea

Then you need to change the rule of metaclass of your class, to let it only allow create the instance once only.
If the class instance is not None (instance is existed), then return the instance, else create new instance of class.

One more important thing, metaclass is type
class is the instance of type (metaclass)
the origin of class is type (I may say that)
python use type to instantiate a new class

Here is some useful reference:
https://stackoverflow.com/questions/100003/what-is-a-metaclass-in-python

singleton.py

class Singleton(type):
    def __init__(cls, name, bases, dict):
        super(Singleton, cls).__init__(name, bases, dict)
        cls._instance = None

    def __call__(cls, *args, **kw):
        if cls._instance is None:
            cls._instance = super(Singleton, cls).__call__(*args, **kw)
        return cls._instance


class LempConfHelper(object):
    __metaclass__ = Singleton