Single instance of class in Python

Question

I am creating a Python application that includes socket communication with a server. I would like to have a module which can be used throughout my entire application (several other modules). Currently my module look like this:

class SocketCommunication:

    def __init__(self):
        self.socketIO = SocketIO(settings.ADDRESS, settings.PORT, Namespace)

    def emit(self, message, data):
        json_data = json.dumps(data.__dict__)
        self.socketIO.emit(message, json_data)


class Namespace(BaseNamespace):
    def on_connect(self):
        print '[Connected]'

    def on_disconnect(self):
        print "[Disconnected]"

When I use this in other modules I do the following:

import SocketCommunication
self.sc = SocketCommunication()

The problem is that every time I do this, a new connection is created which will be displayed as a new client on the server, and that is undesirable. From what I can read, Singletons should be avoided in Python and therefore I am curious about what is best practice for this type of problem?

Answer 1

Singletons are controversial because they're often used as a way to wrap up global variables. This is why some people advocate for their avoidance. Globals make testing harder, they limit access control, and often lead to strong coupling between variables. (see http://wiki.c2.com/?GlobalVariablesAreBad for more details as to why globals are generally bad practice)

In your particular scenario, using a singleton is most likely appropriate because you are simply trying to stop SocketCommunication being initialised multiple times (for good reason), rather than trying to use it as a container for a global state.

See Are Singletons really that bad? and What is so bad about singletons? for some discussion on Singletons.

Answer 2

The following are three ways to use singleton in Python. Using metaclass and decorator to reach the goal.

1. use __new__

   class Singleton(object):
       def __new__(cls, *args, **kw):
           if not hasattr(cls, '_instance'):
               orig = super(Singleton, cls)
               cls._instance = orig.__new__(cls, *args, **kw)
           return cls._instance
   
   class MyClass(Singleton):
       a = 1
   
   one = MyClass()
   two = MyClass()
   
   two.a = 3
   print one.a
   #3
   print id(one)
   #29097904
   print id(two)
   #29097904
   print one == two
   #True
   print one is two
   #True
   

2. use __metaclass__

   class Singleton2(type):
       def __init__(cls, name, bases, dict):
           super(Singleton2, cls).__init__(name, bases, dict)
           cls._instance = None
   
   def __call__(cls, *args, **kw):
       if cls._instance is None:
           cls._instance = super(Singleton2, cls).__call__(*args, **kw)
       return cls._instance
   
   
   class MyClass2(object):
       __metaclass__ = Singleton2
   
   one = MyClass2()
   two = MyClass2()
   
   two.a = 3
   print one.a
   #3
   print id(one)
   #31495472
   print id(two)
   #31495472
   print one == two
   #True
   print one is two
   #True
   

3. use decorator

    def singleton(cls, *args, **kw):
       instances = {}
       def _singleton():
          if cls not in instances:
               instances[cls] = cls(*args, **kw)
          return instances[cls]
       return _singleton
   
   @singleton
   class MyClass3(object):
       a = 1
       def __init__(self, x=0):
           self.x = x
   
   one = MyClass3()
   two = MyClass3()
   
   two.a = 3
   print one.a
   #3
   print id(one)
   #29660784
   print id(two)
   #29660784
   print one == two
   #True
   print one is two
   #True
   one.x = 1
   print one.x
   #1
   print two.x
   #1
   

---

I prefer to use decorator.