It's pretty common for extension code to try to get the instance of the

extension from places where the instance isn't passed. The common

pattern is to import the extension class, the extension manager, and to

do a fetch. While that works, it's annoying.
Since an extension can only be instantiated once, it's pretty safe to

store a handle to the instance on the class. Now,

ExtensionSubclass.instance will point to the instance when the extension

is enabled.
Extensions may still need to deal with circular imports to get the class

(depending on where it's calling this from), but that's not any

different from before, and it's much nicer than dealing with the

extension manager.

Made use of this in an extension I'm building. I was able to fetch the

instance from a form that didn't have the extension passed.