The Singleton Design Pattern

The singleton is a pattern where a class is created with the anticipation of only ever being instantiated once¹. This enables the class to be directly synonymous with it's instantiation; or perhaps for the instantiation to be an interface for the class. When the singleton instance is accessible globally (which seems to be the usual case) the singleton provides a sort of lazy-loadable global that doesn't actually pollute global space either literally or conceptually.

At a glance, singletons:

• Are classes designed to have only once instance.
• Are (generally) easily accessible from anywhere in a codebase.

Interesting Examples of Singletons

• Python modules.
• Many flask extensions.
• The flask app itself, under most circumstances.

Footnotes

1. In fact, classes may not be neccessary at all. More research needed - JRR Nov 24