python 實現單例模式的5種方法

阿新 • • 發佈：2020-09-26

一、classmethod裝飾器

# 全域性變數
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__)

輸出結果

<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、類的裝飾器

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,port):
    self.ip = ip
    self.port = port
 
 
obj1 = MySQL1()
obj2 = MySQL1()
obj3 = MySQL1('1.1.1.3',8080)
print(obj1)
print(obj2,obj3.__dict__)

執行結果

<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、元類

class Mymetaclass(type):
  def __init__(self,class_name,class_bases,class_dic):
    super().__init__(class_name,class_dic)
    self.__instance = self(ip,port)
 
  def __call__(self,**kwargs):
    if args or kwargs:
      obj = self.__new__(self)
      self.__init__(obj,**kwargs)
      self.__instance = obj
    return self.__instance
 
 
class MySQL2(metaclass=Mymetaclass):
  def __init__(self,port):
    self.ip = ip
    self.port = port
 
 
obj1 = MySQL2()
obj2 = MySQL2()
obj3 = MySQL2('1.1.1.3',80)
print(obj1)
print(obj2,obj3.__dict__)

執行結果

<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、模組匯入

# instance.py
 
class MySQL:
  def __init__(self,port):
    self.ip = ip
    self.port = port
 
 
ip = '192.168.13.98'
port = 3306
instance = MySQL(ip,port)
 
 
# 測試程式碼
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',obj3.__dict__)

執行結果

<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、重寫__new__()

class MySQL3(object):
  __instance = None
  __first_init = True
 
  def __init__(self,port):
    if self.__first_init:
      self.ip = ip
      self.port = port
      self.__first_init = False
 
  def __new__(cls,**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',obj3.__dict__)

執行結果

<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','_MySQL3__first_init': False}

注：前四種可以實現單例模式，但都不是絕對單例模式，可以建立新的物件，但是第五種方式是絕對單例模式，全域性只能真正建立一次物件

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