Python单例（Singleton）

Python语言可使用类修饰符实现单例。

def singleton(cls):
    _instance = None
    def wrapper():
        nonlocal _instance
        if _instance is None:
            _instance = cls()
        return _instance
    return wrapper

@singleton   
class cls1:
    def __init__(self):
        pass
    
a=cls1()
b=cls1()
print(a is b)

输出结果为：

True

修饰符（decorator）

以下引自Python documents。

A function returning another function, usually applied as a function transformation using the @wrapper syntax.

The decorator syntax is merely syntactic sugar, the following two function definitions are semantically equivalent:

def f(...):
    ...
f = staticmethod(f)

@staticmethod
def f(...):
    ...