Singleton
The singleton pattern restricts the instantiation of a class to one object, which is useful when you need one object to coordinate actions for the system.
The basic idea is that only one instance of a particular class, doing a job, is created for the needs of the program. To ensure that this works, we need mechanisms that prevent the instantiation of the class more than once and also prevent cloning.
Real-world examples
In a real-life scenario, we can think of the captain of a ship or a boat. On the ship, he is the one in charge. He is responsible for important decisions, and a number of requests are directed to him because of this responsibility.
In software, the Plone CMS has, at its core, an implementation of the singleton. There are actually several singleton objects available at the root of a Plone site, called tools, each in charge of providing a specific set of features for the site. For example, the Catalog tool deals with content indexation and search features (built-in search...
