The Monostate Singleton pattern

We discussed the Gang of Four and their book in Chapter 1, Introduction to Design Patterns. GoF's Singleton design pattern says that there should be one and only one object of a class. However, as per Alex Martelli, typically what a programmer needs is to have instances sharing the same state. He suggests that developers should be bothered about the state and behavior rather than the identity. As the concept is based on all objects sharing the same state, it is also known as the Monostate pattern.

The Monostate pattern can be achieved in a very simple way in Python. In the following code, we assign the __dict__ variable (a special variable of Python) with the __shared_state class variable. Python uses __dict__ to store the state of every object of a class. In the following code, we intentionally assign __shared_state to all the created instances. So when we create two instances, 'b' and 'b1', we get two different objects unlike Singleton where we have just one object. However, the object states, b.__dict__ and b1.__dict__ are the same. Now, even if the object variable x changes for object b, the change is copied over to the __dict__ variable that is shared by all objects and even b1 gets this change of the x setting from one to four:

class Borg:
    __shared_state = {"1":"2"}
    def __init__(self):
        self.x = 1
        self.__dict__ = self.__shared_state
        pass

b = Borg()
b1 = Borg()
b.x = 4

print("Borg Object 'b': ", b) ## b and b1 are distinct objects
print("Borg Object 'b1': ", b1)
print("Object State 'b':", b.__dict__)## b and b1 share same state
print("Object State 'b1':", b1.__dict__)

The following is the output of the preceding snippet:

Another way to implement the Borg pattern is by tweaking the __new__ method itself. As we know, the __new__ method is responsible for the creation of the object instance:

class Borg(object):
     _shared_state = {}
     def __new__(cls, *args, **kwargs):
       obj = super(Borg, cls).__new__(cls, *args, **kwargs)
       obj.__dict__ = cls._shared_state
       return obj