Python 3 Object-Oriented Programming - Third Edition

State transition as coroutines

The state pattern is the canonical object-oriented solution to state-transition problems. However, you can get a similar effect by constructing your objects as coroutines. Remember the regular expression log file parser we built in Chapter 9, The Iterator Pattern? That was a state-transition problem in disguise. The main difference between that implementation and one that defines all the objects (or functions) used in the state pattern is that the coroutine solution allows us to encode more of the boilerplate in language constructs. There are two implementations, but neither one is inherently better than the other. The state pattern is actually the only place I would consider using coroutines outside of asyncio ...