class Singleton(object):
    def __init__(self):
        self._val = 10
    @property
    def val(self):
        return self._val
    @val.setter
    def val(self, val):
        self._val = val
singleton = Singleton()

from Singleton import singleton
assert singleton.val == 10
singleton.val = 20
assert singleton.val == 20

class Singleton(type):
    def __init__(cls, what, bases, dict):
        super().__init__(what, bases, dict)
        cls._instance = None
    def __call__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super().__call__(*args, **kwargs)
        return cls._instance
class MyClass(metaclass=Singleton):
    pass
class1 = MyClass()
class2 = MyClass()
assert class1 == class2

class Singleton(object):
    _instance = None
    def __new__(cls, *more):
        if not cls._instance:
            cls._instance = super(Singleton, cls).__new__(cls, *more)
        return cls._instance
class MyClass(Singleton):
    pass
class1 = MyClass()
class2 = MyClass()
assert id(class1) == id(class2)

import functools
def singleton(cls):
    cls.__new_original__ = cls.__new__
    @functools.wraps(cls.__new__)
    def singleton_new(cls, *args, **kw):
        it = cls.__dict__.get(‘__it__‘)
        if it is not None:
            return it
        cls.__it__ = it = cls.__new_original__(cls, *args, **kw)
        it.__init_original__(*args, **kw)
        return it
    cls.__new__ = singleton_new
    cls.__init_original__ = cls.__init__
    cls.__init__ = object.__init__
    return cls
@singleton
class myclass:
    pass
class1 = myclass()
class2 = myclass()
assert class1 == class2