[Python知识库]python --单例实现

使用函数装饰器实现单例

def singleton(cls):
    _instance = {}

    def inner():
        if cls not in _instance:
            _instance[cls] = cls()
        return _instance[cls]
    return inner
    
@singleton
class Cls(object):
    def __init__(self):
        pass

cls1 = Cls()
cls2 = Cls()
print(id(cls1) == id(cls2))

使用类装饰器实现单例

class Singleton(object):
    def __init__(self, cls):
        self._cls = cls
        self._instance = {}

    def __call__(self):
        if self._cls not in self._instance:
            self._instance[self._cls] = self._cls()
        return self._instance[self._cls]


@Singleton
class Cls2(object):
    def __init__(self):
        pass


cls1 = Cls2()
cls2 = Cls2()
print(id(cls1) == id(cls2))

同时，由于是面对对象的，这里还可以这么用

class Cls3():
    pass

Cls3 = Singleton(Cls3)
cls3 = Cls3()
cls4 = Cls3()
print(id(cls3) == id(cls4))

使用 new 关键字实现单例模式

class Single(object):
    _instance = None
    def __new__(cls, *args, **kw):
        if cls._instance is None:
            cls._instance = object.__new__(cls, *args, **kw)
        return cls._instance
    def __init__(self):
        pass

single1 = Single()
single2 = Single()
print(id(single1) == id(single2))

使用 metaclass 实现单例模式

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


class Cls4(metaclass=Singleton):
    pass


cls1 = Cls4()
cls2 = Cls4()
print(id(cls1) == id(cls2))

使用类(多线程)

import time
import threading


class Singleton(object):
    _instance_lock = threading.Lock()

    def __init__(self):
        time.sleep(1)

    @classmethod
    def instance(cls, *args, **kwargs):
        if not hasattr(Singleton, "_instance"):
            with Singleton._instance_lock:
                if not hasattr(Singleton, "_instance"):
                    Singleton._instance = Singleton(*args, **kwargs)
        return Singleton._instance


def task(arg):
    obj = Singleton.instance()
    print(obj)


for i in range(10):
    t = threading.Thread(target=task, args=[i, ])
    t.start()
time.sleep(20)
obj = Singleton.instance()
print(obj)

这种方式实现的单例模式，使用时会有限制，以后实例化必须通过 obj = Singleton.instance()

如果用 obj=Singleton() ,这种方式得到的不是单例

new方法(多线程)

import threading


class Singleton(object):
    _instance_lock = threading.Lock()

    def __init__(self):
        pass

    def __new__(cls, *args, **kwargs):
        if not hasattr(Singleton, "_instance"):
            with Singleton._instance_lock:
                if not hasattr(Singleton, "_instance"):
                    Singleton._instance = object.__new__(cls)
        return Singleton._instance


obj1 = Singleton()
obj2 = Singleton()
print(obj1, obj2)


def task(arg):
    obj = Singleton()
    print(obj)


for i in range(10):
    t = threading.Thread(target=task, args=[i, ])
    t.start()

元类(多线程)

import threading

class SingletonType(type):
    _instance_lock = threading.Lock()
    def __call__(cls, *args, **kwargs):
        if not hasattr(cls, "_instance"):
            with SingletonType._instance_lock:
                if not hasattr(cls, "_instance"):
                    cls._instance = super(SingletonType,cls).__call__(*args, **kwargs)
        return cls._instance

class Foo(metaclass=SingletonType):
    def __init__(self,name):
        self.name = name


obj1 = Foo('name')
obj2 = Foo('name')
print(obj1,obj2)

new(另一种写法)

class Singleton(object):
  
  def __new__(cls, *args, **kwargs):
    if not hasattr(cls, '_instance'):
      orig = super(Singleton, cls)
      cls._instance = orig.__new__(cls, *args, **kwargs)
    return cls._instance
  
class MyClass(Singleton):
  a = 1
  
one = MyClass()
two = MyClass()
  
#one和two完全相同,可以用id(), ==, is检测
print id(one)  # 29097904
print id(two)  # 29097904
print one == two  # True
print one is two  # True

元类(另一种写法)

class Singleton2(type):
  
  def __init__(cls, name, bases, dict):
    super(Singleton2, cls).__init__(name, bases, dict)
    cls._instance = None
  
  def __call__(cls, *args, **kwargs):
    if cls._instance is None:
      cls._instance = super(Singleton2, cls).__call__(*args, **kwargs)
    return cls._instance
  
class MyClass2(object):
  __metaclass__ = Singleton2
  a = 1
  
one = MyClass2()
two = MyClass2()
  
print id(one)  # 31495472
print id(two)  # 31495472
print one == two  # True
print one is two  # True