python 实现单例模式的5种方法

一、classmethod装饰器

# 全局变量
ip = '192.168.13.98'
port = '3306'
class MySQL:
  __instance = None

  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

  @classmethod
  def instance(cls, *args, **kwargs):
    if args or kwargs:
      cls.__instance = cls(*args, **kwargs)
    return cls.__instance

obj1 = MySQL.instance(ip, port)
obj2 = MySQL.instance()
obj3 = MySQL.instance()
print(obj1)
print(obj2, obj2.__dict__)
print(obj3, obj3.__dict__)

输出结果

<main.MySQL object at 0x058D6F30>
<main.MySQL object at 0x058D6F30> {'ip': '192.168.13.98', 'port': '3306'}
<main.MySQL object at 0x058D6F30> {'ip': '192.168.13.98', 'port': '3306'}

二、类的装饰器

def singlegon(cls):
  _instance = cls(ip, port)

  def wrapper(*args, **kwargs):
    if args or kwargs:
      return cls(*args, **kwargs)
    return _instance

  return wrapper

@singlegon
class MySQL1:
  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

obj1 = MySQL1()
obj2 = MySQL1()
obj3 = MySQL1('1.1.1.3', 8080)
print(obj1)
print(obj2, obj2.__dict__)
print(obj3, obj3.__dict__)
　　

运行结果

<main.MySQL1 object at 0x04C102B0>
<main.MySQL1 object at 0x04C102B0> {'ip': '192.168.13.98', 'port': '3306'}
<main.MySQL1 object at 0x04C10310> {'ip': '1.1.1.3', 'port': 8080}

三、元类

class Mymetaclass(type):
  def __init__(self, class_name, class_bases, class_dic):
    super().__init__(class_name, class_bases, class_dic)
    self.__instance = self(ip, port)

  def __call__(self, *args, **kwargs):
    if args or kwargs:
      obj = self.__new__(self)
      self.__init__(obj, *args, **kwargs)
      self.__instance = obj
    return self.__instance

class MySQL2(metaclass=Mymetaclass):
  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

obj1 = MySQL2()
obj2 = MySQL2()
obj3 = MySQL2('1.1.1.3', 80)
print(obj1)
print(obj2, obj2.__dict__)
print(obj3, obj3.__dict__)

运行结果

<main.MySQL2 object at 0x04D003B0>
<main.MySQL2 object at 0x04D003B0> {'ip': '192.168.13.98', 'port': '3306'}
<main.MySQL2 object at 0x04D003D0> {'ip': '1.1.1.3', 'port': 80}

四、模块导入

# instance.py

class MySQL:
  def __init__(self, ip, port):
    self.ip = ip
    self.port = port

ip = '192.168.13.98'
port = 3306
instance = MySQL(ip, port)

# 测试代码
import os, sys

sys.path.append(os.path.dirname(os.path.dirname(__file__)))
from test import instance

obj1 = instance.instance
obj2 = instance.instance
obj3 = instance.MySQL('1.1.1.3', 80)
print(obj1)
print(obj2, obj2.__dict__)
print(obj3, obj3.__dict__)

运行结果

<day30.instance.MySQL object at 0x052B0AB0>
<day30.instance.MySQL object at 0x052B0AB0> {'ip': '192.168.13.98', 'port': 3306}
<day30.instance.MySQL object at 0x052B03F0> {'ip': '1.1.1.3', 'port': 80}

五、重写__new__()

class MySQL3(object):
  __instance = None
  __first_init = True

  def __init__(self, ip, port):
    if self.__first_init:
      self.ip = ip
      self.port = port
      self.__first_init = False

  def __new__(cls, *args, **kwargs):
    if not cls.__instance:
      cls.__instance = object.__new__(cls)
    return cls.__instance

obj1 = MySQL3(ip, port)
obj2 = MySQL3(ip, port)
obj3 = MySQL3('1.1.1.3', 80)
print(obj1)
print(obj2, obj2.__dict__)
print(obj3, obj3.__dict__)

运行结果

<main.MySQL3 object at 0x059603F0>
<main.MySQL3 object at 0x059603F0> {'ip': '192.168.13.98', 'port': '3306', '_MySQL3__first_init': False}
<main.MySQL3 object at 0x059603F0> {'ip': '192.168.13.98', 'port': '3306', '_MySQL3__first_init': False}

注：前四种可以实现单例模式，但都不是绝对单例模式，可以创建新的对象，但是第五种方式是绝对单例模式，全局只能真正创建一次对象

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