Singletone examples in python

singleton_python.py

# One
class Singleton:
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, 'instance'):
            cls.instance = super(Singleton, cls).__new__(cls)
        return cls.instance


a = Singleton()
a.prop = "PROP"
print(a.prop)

b = Singleton()
print(b.prop)

b.prop = "NEW"
print(a.prop)

assert a is b


# Two
class BorgSingleton:
    _shared_state = {}

    def __init__(self):
        self.__dict__ = self._shared_state


c = BorgSingleton()
c.prop = "PROP"
print(c.prop)

d = BorgSingleton()
print(d.prop)

d.prop = "NEW"
print(c.prop)

assert a is b


# Three
class SingletonMetaclass(type):
    instance = None

    def __call__(cls, *args, **kwargs):
        if cls.instance is None:
            cls.instance = super(SingletonMetaclass, cls).__call__(*args, **kwargs)
        return cls.instance


class MSingleton(metaclass=SingletonMetaclass):
    pass


a = MSingleton()
a.prop = "PROP"
print(a.prop)

b = MSingleton()
print(b.prop)

b.prop = "NEW"
print(a.prop)

assert a is b


# Four
class AdHookSingleton:
    __instance = None

    @classmethod
    def getInstance(cls):
        if not cls.__instance:
            cls.__instance = AdHookSingleton()
        return cls.__instance


a = AdHookSingleton().getInstance()
a.prop = "PROP"
print(a.prop)

b = AdHookSingleton().getInstance()
print(b.prop)

b.prop = "NEW"
print(a.prop)

assert a is b


# Five
# You can create a module (modules are singletone by default in Python)

# https://hub.packtpub.com/python-design-patterns-depth-singleton-pattern/
# https://www.oreilly.com/library/view/python-cookbook/0596001673/ch05s23.html
# https://gist.github.com/nmariz/3892000
# https://stackoverflow.com/questions/6760685/creating-a-singleton-in-python
# https://webdevblog.ru/realizaciya-shablona-singleton-v-python/