单例模式链接数据库

 

参考了文章:https://www.cnblogs.com/linxiyue/p/3902256.html

http://www.cnblogs.com/liujianzuo888/articles/5561076.html

主要思想是利用类方法或者类属性创建实例,在创建之前做一个判断:

1)使用__new__ 方法

import pymysql


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

    def __init__(self):
        self.conn = pymysql.connect(host='127.0.0.1', user='root', password='root', database='job',
                                    charset='utf8')


if __name__ == '__main__':
    a = Singleton()
    b = Singleton()
    print(a.conn, b.conn)

运行输出结果是:

可以看到两个连接都是同一个实例

但是我有疑问,如果实例化要传参,调用__init__初始化的,老是报错,求大神们解答

import pymysql


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


class A(Singleton):
    def __init__(self, name):
        self.conn = pymysql.connect(host='127.0.0.1', user='root', password='root', database='job',
                                    charset='utf8')
        self.name = name


if __name__ == '__main__':
    apple = A('apple')
    banana = A('banana')

Traceback (most recent call last):
  File "/Users/daniel/Documents/test/singleton.py", line 19, in 
    apple = A('apple')
  File "/Users/daniel/Documents/test/singleton.py", line 7, in __new__
    cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
TypeError: object() takes no parameters

另一种是用方法定义

class Singleton(object):
    _instance = None

    def __init__(self, name):
        self.name = name

    @classmethod
    def get_instance(cls, name):
        if cls._instance:
            return cls._instance
        else:
            obj = cls(name)
            cls._instance = obj
            return obj


if __name__ == '__main__':
    _a = Singleton.get_instance('apple')
    _b = Singleton.get_instance('banana')
    print(_a, _b)

结果如下:

<__main__.Singleton object at 0x10205bd30> <__main__.Singleton object at 0x10205bd30>

总觉得用类方法感觉不够统一,能不能解决__new__创建单例传参的问题呢?可能是py2 和py3的区别吧

参考了文章:

https://docs.lvrui.io/2016/07/03/Python%E5%AE%9E%E7%8E%B0%E7%9A%84%E5%8D%95%E4%BE%8B%E6%A8%A1%E5%BC%8F/

代码如下:

import pymysql


class Singleton(object):
    _instance = None

    def __new__(cls, name):
        if not cls._instance:
            cls._instance = object.__new__(cls)
        return cls._instance


class A(Singleton):
    def __init__(self, name):
        self.name = name
        self.conn = pymysql.connect(host='127.0.0.1', user='root', password='root', database='job',
                                    charset='utf8')


if __name__ == '__main__':
    a = A('apple')
    b = A('bnana')
    print(a, b)

执行结果:

<__main__.A object at 0x10205bf28> <__main__.A object at 0x10205bf28>