打日志 :单例, _init_flag

背景 非单例

class A:
    pass
a1=A()
a2=A()
print(id(a1))
print(id(a2))
'''
1618986824272
1618986826096
a1和a2的id不一样
'''

 单例

class Single:
    _self = None
    def __new__(cls, *args, **kwargs):
        if cls._self is None:
            cls._self=super().__new__(cls)
        return cls._self
    def __init__(self):
        print(".....")
s1=Single()
s2=Single()
print(id(s1))
print(id(s2))
'''
.....
2252706872672
2252706872672
两个示例的id是一样的
'''

 单例  且init 限定仅一次

class A:
    pass
a1=A()
a2=A()
print(id(a1))
print(id(a2))
'''
1618986824272
1618986826096
'''
class Single:
    _self = None
    # 定义初始化的次数为1次；没定义前  实例化几次，init几次
    _init_flag=True
    def __new__(cls, *args, **kwargs):
        if cls._self is None:
            cls._self=super().__new__(cls)
        return cls._self
    def __init__(self):
        if self._init_flag:# 如果是真，说明没被init过，就init
            print(".....")
            self._init_flag=False # 打印后就设置为假


s1=Single()
s2=Single()
print(id(s1))
print(id(s2))
'''
.....
.....
2252706872672
2252706872672
'''