Python实现单例模式的四种方式详解

# -*- coding: utf-8 -*-
# time: 2022/5/17 10:31
# file: singleton.py
# author: tom
# 公众号: 玩转测试开发


class Singleton(object):
    def __init__(self, name):
        self.name = name

    def run(self):
        print(self.name)

s = Singleton("Tom")

# -*- coding: utf-8 -*-
# time: 2022/5/17 10:51
# file: test_singleton.py
# author: tom
# 公众号: 玩转测试开发
from singleton import s as s1
from singleton import s as s2


# Method One：通过导入模块实现
def show_method_one():
    """

    :return:
    """
    print(s1)
    print(s2)
    print(id(s1))
    print(id(s2))


show_method_one()


# Method Two：通过装饰器实现
def singleton(cls):
    # 创建一个字典用来保存类的实例对象
    _instance = {}

    def _singleton(*args, **kwargs):
        # 先判断这个类有没有对象
        if cls not in _instance:
            _instance[cls] = cls(*args, **kwargs)  # 创建一个对象,并保存到字典当中
        # 将实例对象返回
        return _instance[cls]

    return _singleton


@singleton
class Demo2(object):
    a = 1

    def __init__(self, x=0):
        self.x = x


a1 = Demo2(1)
a2 = Demo2(2)
print(id(a1))
print(id(a2))


# Method Three：通过使用类实现
class Demo3(object):
    # 静态变量
    _instance = None
    _flag = False

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

    def __init__(self):
        if not Demo3._flag:
            Demo3._flag = True


b1 = Demo3()
b2 = Demo3()
print(id(b1))
print(id(b2))


# Method Four：通过__new__ 方法实现
class Demo4:
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_instance'):
            cls._instance = super(Demo4, cls).__new__(cls)
        return cls._instance


c1 = Demo4()
c2 = Demo4()
print(id(c1))
print(id(c2))