python中的四种单例模式

 1 # 单例模式：无法支持多线程情况
 2 """
 3 class Singleton(object):
 4     def __init__(self):
 5         import time
 6         time.sleep(1)
 7     @classmethod
 8     def instance(cls, *args, **kwargs):
 9         if not hasattr(Singleton, "_instance"):
10             Singleton._instance = Singleton(*args, **kwargs)
11         return Singleton._instance
12 import threading
13 
14 def task(arg):
15     obj = Singleton.instance()
16     print(obj)
17 
18 for i in range(10):
19     t = threading.Thread(target=task,args=[i,])
20     t.start()

第一种

 1 import time
 2 import threading
 3 class Singleton(object):
 4     _instance_lock = threading.Lock()
 5     def __init__(self):
 6         time.sleep(4)
 7     @classmethod
 8     def instance(cls, *args, **kwargs):
 9         if not hasattr(Singleton, "_instance"):
10             with Singleton._instance_lock:
11                 if not hasattr(Singleton, "_instance"):
12                     Singleton._instance = Singleton(*args, **kwargs)
13         return Singleton._instance
14 def task(arg):
15     obj = Singleton.instance()
16     print(obj)
17 for i in range(10):
18     t = threading.Thread(target=task,args=[i,])
19     t.start()
20 obj = Singleton.instance()
21 print(obj)

第二种

 1 import time
 2 import threading
 3 class Singleton(object):
 4     _instance_lock = threading.Lock()
 5     def __init__(self):
 6         pass
 7     def __new__(cls, *args, **kwargs):
 8         if not hasattr(Singleton, "_instance"):
 9             with Singleton._instance_lock:
10                 if not hasattr(Singleton, "_instance"):
11                     Singleton._instance = object.__new__(cls, *args, **kwargs)
12         return Singleton._instance
13 obj1 = Singleton()
14 obj2 = Singleton()
15 print(obj1,obj2)

第三种

 1 import threading
 2 class SingletonType(type):
 3     _instance_lock = threading.Lock()
 4     def __call__(cls, *args, **kwargs):
 5         if not hasattr(cls, "_instance"):
 6             with SingletonType._instance_lock:
 7                 if not hasattr(cls, "_instance"):
 8                     cls._instance = super(SingletonType,cls).__call__(*args, **kwargs)
 9         return cls._instance
10 class Foo(metaclass=SingletonType):
11     def __init__(self,name):
12         self.name = name
13 obj1 = Foo('name')
14 obj2 = Foo('name')
15 print(obj1,obj2)

第四种