單例模式：　顧名思義，一個類，在整個專案的執行週期內只有一個例項

def singleton(cls,*args,**kwargs):
    instance = {}
    def _singleton():
        if cls not in instance:
            instance[cls] = cls(*args,**kwargs)
        return instance[cls]
    return _singleton

@singleton

class test_singleton(object):
    def __init__(self):
        self.sum = 0
    def add(self):
        self.num = 100

cls1 = test_singleton()
cls2 = test_singleton()
print(id(cls1)==id(cls2))

利用__new__實現單例子

class A(object):
    __instance = None

    def __new__(cls, *args, **kwargs):
        if cls.__instance is None:
            cls.__instance = object.__new__(cls,*args,**kwargs)
        return cls.__instance

    def __init__(self):
        self.value = 3
a = A()
a.value = 2
print(a.value)
b = A()
print(a is b)
print(a.value)
print(b.value)