In python singleton

1, what is the singleton:

　　Refers to a singleton pattern class has only one instance of the object, create an instance of the object, the instance is created and then a return object reference. (Simply say that two instances of objects same ID, these saving memory space)

2, Singleton pattern created:　

　　For example create a class, such as the universe is only one Earth to help understand the singleton

class Earth:
    pass
a=Earth()
print(a)
B = Earth ()
 Print (B)
 ***** ****** Results
32096552
32096496 
we can see two examples are not the same memory ID, the class is not a singleton

　　So how can let the class to create only one instance, and then create instances is to return the last reference to the object it?

　　We understand, python, a class object instance is created by calling the parent object of __new __ (cls) method to create an object

　　We can go to the implementation class by overriding __new __ (cls) method to create only one instance:

class Earth (Object):
     __instance = None
     DEF  __new__ (CLS):
         IF CLS. __instance == None: # if __instance empty prove for the first time to create an instance 
             . CLS __instance . = Object __new__ (CLS)
              return CLS. __instance 
        the else :
             # If it is not the first time to create an instance, return to the previous object reference 
             return . CLS __instance   
a = Earth ()
 Print (the above mentioned id (a))
B = Earth ()
 Print (B)
 ****** ***** Results
1730389200
1730389200

　　The above example we create a singleton class by __new__ method, but the singleton class and then use multithreading is there will be some problems. We need to add mutex way to solve the problem.

import threading
class Earth(object):
    _instance_lock=threading.Lock()
    def __init__(self):
         pass
    def __new__(cls,*args,**kwargs):
        if not hasattr(Earth,'_instance'):
            with Earth._instance_lock:
                 if not hasattr(Earth,'_instance'):
                      Earth._instance=object.__new__(cls)
        return Earth._instance
a=Earth()
b=Earth()
print(a,b)
def task(arg):
    obj=Earth()
    print(obj)
for i in range(10):
    t=threading.Thread(target=task,args=[i,])
    t.start()
*****result*****
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>
<__main__.Singleton object at 0x00000000029D06A0>

3, other methods for creating the single Example: 3, the use of class 1, using the module, the decorator 4 2, based on the way metaclass

Specific reference blog: https://www.cnblogs.com/huchong/p/8244279.html

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