python 单例模式
03 November 2018
Python 单例实现方法
python 实现单例方法有多种，下面我们介绍几种常用的方法。
1. 使用元类实现单例
- 创建一个 SingleInstance 的元类
- 类指定 metaclass = SingleInstance
# 创建一个 metaclass 类
class SingleInstance(type):
"""
类创建对象的时候会调用该方法, 返回类对象实例
"""
def __call__(cls, *args, **kwargs):
instance = getattr(cls, '__instance__', None)
if instance is None:
instance = super(SingleInstance, cls).__call__(*args, **kwargs)
setattr(cls, '__instance__', instance)
print("create instance ", instance)
else:
print("use old instance ", instance)
print('got instance for ', cls)
return instance
"""
测试方法：
1. 每次实验创建两个类，确认不同的类之间不会相互影响
2. 每个类创建两个对象，确认两个对象是一样的
"""
class TestClass0(metaclass=SingleInstance):
def __init__(self):
print("init test0 class object")
class TestClass1(metaclass=SingleInstance):
def __init__(self):
print("init test1 class object")
# 测试第一个类对象
test00 = TestClass0()
test01 = TestClass0()
print(test00, test01)
print('\n----------------------\n')
# 测试第二个类对象
test10 = TestClass1()
test11 = TestClass1()
print(test10, test11)
init test0 class object
create instance <__main__.TestClass0 object at 0x7f5492fe66a0>
got instance for <class '__main__.TestClass0'>
use old instance <__main__.TestClass0 object at 0x7f5492fe66a0>
got instance for <class '__main__.TestClass0'>
<__main__.TestClass0 object at 0x7f5492fe66a0> <__main__.TestClass0 object at 0x7f5492fe66a0>
----------------------
init test1 class object
create instance <__main__.TestClass1 object at 0x7f5492fe69b0>
got instance for <class '__main__.TestClass1'>
use old instance <__main__.TestClass1 object at 0x7f5492fe69b0>
got instance for <class '__main__.TestClass1'>
<__main__.TestClass1 object at 0x7f5492fe69b0> <__main__.TestClass1 object at 0x7f5492fe69b0>
2. 通过类的 new 方法实现
- 第一次创建对象并保存到类属性中
- 后续从类属性中取出对象，不再创建新的对象
class SingleClass():
def __new__(cls, *args, **kwargs):
print('new instance for ', cls)
if not hasattr(cls, '__instance__'):
print('create instance for ', cls)
instance = super(SingleClass, cls).__new__(cls, *args, **kwargs)
cls.__instance__ = instance
return cls.__instance__
class SubClass(SingleClass):
pass
test00 = SingleClass()
test01 = SingleClass()
print(test00, test01)
print('\n----------------------\n')
test10 = SubClass()
test11 = SubClass()
print(test10, test11)
new instance for <class '__main__.SingleClass'>
create instance for <class '__main__.SingleClass'>
new instance for <class '__main__.SingleClass'>
<__main__.SingleClass object at 0x7f5492fc75f8> <__main__.SingleClass object at 0x7f5492fc75f8>
----------------------
new instance for <class '__main__.SubClass'>
new instance for <class '__main__.SubClass'>
<__main__.SingleClass object at 0x7f5492fc75f8> <__main__.SingleClass object at 0x7f5492fc75f8>
我们发现这样实现是有问题的，SubClass 无法创建自己的单例。 因为父类 SingleClass 的类对象数据会被所有子类共享，导致所有 SingleClass 子类无法创建新的对象。
解决方法是我们在 SingleClass 类中使用字典保存每个子类的实例对象，改进后代码如下：
class SingleTClass():
__instance__ = {}
def __new__(cls, *args, **kwargs):
print('new instance for ', cls)
if not cls in cls.__instance__:
print('create instance for ', cls)
instance = super(SingleTClass, cls).__new__(cls, *args, **kwargs)
cls.__instance__[cls] = instance
return cls.__instance__[cls]
class SubTClass(SingleTClass):
pass
test00 = SingleTClass()
test01 = SingleTClass()
print(test00, test01)
print('\n----------------------\n')
test10 = SubTClass()
test11 = SubTClass()
print(test10, test11)
new instance for <class '__main__.SingleTClass'>
create instance for <class '__main__.SingleTClass'>
new instance for <class '__main__.SingleTClass'>
<__main__.SingleTClass object at 0x7f5492fc7ac8> <__main__.SingleTClass object at 0x7f5492fc7ac8>
----------------------
new instance for <class '__main__.SubTClass'>
create instance for <class '__main__.SubTClass'>
new instance for <class '__main__.SubTClass'>
<__main__.SubTClass object at 0x7f5492fc7438> <__main__.SubTClass object at 0x7f5492fc7438>
3. 通过装饰器实现单例
- 创建一个 singleclass 的装饰器
- 创建类的时候使用上述装饰器
from functools import wraps
"""
把类对象变成一个函数，每次调用类对象创建实例的时候，实际是调用装饰器返回的函数
"""
def singleclass(cls):
__instance__ = {}
@wraps(cls)
def cls_instance(*args, **kwargs):
print('new instance for ', cls)
if not cls in __instance__:
__instance__[cls] = cls(*args, **kwargs)
return __instance__[cls]
return cls_instance
@singleclass
class TestD1Class():
pass
@singleclass
class TestD2Class():
pass
test00 = TestD1Class()
test01 = TestD1Class()
print(test00, test01)
print('\n----------------------\n')
test10 = TestD2Class()
test11 = TestD2Class()
print(test10, test11)
new instance for <class '__main__.TestD1Class'>
new instance for <class '__main__.TestD1Class'>
<__main__.TestD1Class object at 0x7f5492fee518> <__main__.TestD1Class object at 0x7f5492fee518>
----------------------
new instance for <class '__main__.TestD2Class'>
new instance for <class '__main__.TestD2Class'>
<__main__.TestD2Class object at 0x7f5492fee160> <__main__.TestD2Class object at 0x7f5492fee160>