Key 3: Providing the same view to all.
The singleton pattern maintains the same state for all instances of a class. When we change an attribute at one place in a program, it is reflected in all references to this instance. As modules are globally shared, we can use them as singleton methods, and the variables defined in them are the same everywhere. But, there are similar issues in that as the module is reloaded, there may be more singleton classes that are needed. We can also create a singleton pattern using metaclasses in the following manner. The six is a third-party library to help in writing the same code that is runnable on Python 2 and Python 3.
In the following code, Singleton metaclass has a registry dictionary where the instance corresponding to each new class is stored. When any class asks for a new instance, this class is searched for in the registry, and if found, the old instance is passed. Otherwise, a new instance is created, stored in registry, and returned...