Python에서 싱글턴 패턴을 구현하는 방법

class Singleton(object):
    instance = None
    def __init__(self):
        raise SyntaxError('can not instance, please use get_instance')
    def get_instance():
        if Singleton.instance is None:
            Singleton.instance = object.__new__(Singleton)
        return Singleton.instance
a = Singleton.get_instance()
b = Singleton.get_instance()
print('a id=', id(a))
print('b id=', id(b))

class Singleton(object):
    instance = None
    def __init__(self):
        raise SyntaxError('can not instance, please use get_instance')
    def get_instance(cls):
        if Singleton.instance is None:
            Singleton.instance = object.__new__(Singleton)
        return Singleton.instance
a = Singleton.get_instance()
b = Singleton.get_instance()
print('a id=', id(a))
print('b id=', id(b))

class Singleton(object):
    instance = None
    def __init__(self):
        raise SyntaxError('can not instance, please use get_instance')
    def get_instance():
        if Singleton.instance is None:
            Singleton.instance = object.__new__(Singleton)
        return Singleton.instance
a = Singleton.get_instance()
b = Singleton.get_instance()
print(id(a))
print(id(b))

class Singleton(object):
    instance = None
    def __new__(cls, *args, **kw):
        if not cls.instance:
            # cls.instance = object.__new__(cls, *args)
            cls.instance = super(Singleton, cls).__new__(cls, *args, **kw)
        return cls.instance
a = Singleton()
b = Singleton()
print(id(a))
print(id(b))

def Singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance
class MyClass:
    pass
a = MyClass()
b = MyClass()
c = MyClass()
print(id(a))
print(id(b))
print(id(c))

class Singleton(type):
    def __init__(cls, name, bases, dct):
        super(Singleton, cls).__init__(name, bases, dct)
        cls.instance = None
    def __call__(cls, *args):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args)
        return cls.instance
class MyClass(object):
    __metaclass__ = Singleton
a = MyClass()
b = MyClass()
c = MyClass()
print(id(a))
print(id(b))
print(id(c))
print(a is b)
print(a is c)

class Singleton(type):
    def __new__(cls, name, bases, attrs):
        attrs["_instance"] = None
        return super(Singleton, cls).__new__(cls, name, bases, attrs)
    def __call__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instance
class Foo(object):
    __metaclass__ = Singleton
x = Foo()
y = Foo()
print(id(x))
print(id(y))

class Singleton(type):
    def __new__(cls, name, bases, attrs):
        attrs['instance'] = None
        return super(Singleton, cls).__new__(cls, name, bases, attrs)
    def __call__(cls, *args, **kwargs):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
        return cls.instance
class Foo(metaclass=Singleton):
    pass
x = Foo()
y = Foo()
print(id(x))
print(id(y))

class Singleton(object):
    def foo(self):
        print('foo')
    def __call__(self):
        return self
Singleton = Singleton()
Singleton.foo()
a = Singleton()
b = Singleton()
print(id(a))
print(id(b))