python3 singleton

singleton.py

# Method 1. with __new__ magic method
class Singleton:
    def __new__(cls):
        if not hasattr(cls, 'instance'):
            cls.instance = super().__new__(cls)
        return cls.instance

class TestSingletonNew(Singleton):
    pass

# Method 2. with decorator
def singleton(cls, *args, **kwargs):
    instances = {}
    def _singleton():
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return _singleton

@singleton
class TestSingletonDeco:
    pass

# Run Test
tsn1 = TestSingletonNew()
tsn2 = TestSingletonNew()
assert id(tsn1) == id(tsn2)

tsd1 = TestSingletonDeco()
tsd2 = TestSingletonDeco()
assert id(tsd1) == id(tsd2)