python 单例

1 

class Singleton:
    def __new__(cls, *args, **kw):
        if not hasattr(cls, '_instance'):
            cls._instance = object.__new__(cls)
        return cls._instance

one = Singleton()
two = Singleton()

two.a = 3
print(one.a)
# 3
# one和two完全相同,可以用id(), ==, is检测
print(id(one))
# 29097904
print(id(two))
# 29097904
print(one == two)
# True
print(one is two)

2 　　函数装饰器实现：实例化前先调用这个装饰器返回的 inner() 函数，再返回实例

def singleton2(cls):
    _instance = {}  # 新建空字典; 不要_instance = None, 否则 inner 函数中赋值时，_instance 只会在函数作用域搜索，判断时会报错;
    
    def inner():  # 判断是否已有实例,如无,则新建一个实例并返回
        if cls not in _instance:
            _instance[cls] = cls()

        return _instance[cls]  # 返回的是实例对象
    
    return inner

@singleton2
class Cls():
    def __init__(self):
        pass  # 可以照常添加属性和方法
    
cls1 = Cls()
cls2 = Cls()
cls1 is cls2  # True