Python单例模式

单例模式：

装饰器形式：

```
import functools

 

def singleton(cls):

instance = None

 

@functools.wraps(cls)

def getinstance(*args,**kwargs):

nonlocal instance

 

if not instance:

print(args)

print(kwargs)

instance = cls(*args,**kwargs)

return instance

 

return getinstance

 

@singleton

class B:

def __init__(self,url,debug):

self.url = url

self.debug = debug

 

b = B(1,2)

print(id(b),b.__dict__,b.__doc__)

 

b1 = B(10,20)

print(id(b1),b1.__dict__,b1.__doc__)
```

```
new方法形式：
class A:

def __new__(cls, *args, **kwargs):

print('--------')

print(1,cls)

print(2,args)

print(3,kwargs)

if not hasattr(cls,'_instance'):

setattr(cls,'_instance',super().__new__(cls))

setattr(cls,'_count',0)

return cls._instance

 

def __init__(self,url,debug):

print('=======')

if self._count == 0 :

self.url = url

self.debug = debug

 

self.__class__.count = 1

else:

raise Exception('just one instance')

 

def __repr__(self):

return "<B{}{}>".format(self.url,self.debug)

 

b = A(1,debug=2)

print(4,b.__dict__)

print(id(b))

import time

time.sleep(2)

b1= A(2,debug=20)

print(5,b1.__dict__)

print(id(b1))
```