Discussion on answer by andreymal: Устранить циклическую зависимость ссылок на классы в Python

Imported from a comment discussion on https://ru.stackoverflow.com/questions/1545567/%d0%a3%d1%81%d1%82%d1%80%d0%b0%d0%bd%d0%b8%d1%82%d1%8c-%d1%86%d0%b8%d0%ba%d0%bb%d0%b8%d1%87%d0%b5%d1%81%d0%ba%d1%83%d1%8e-%d0%b7%d0%b0%d0%b2%d0%b8%d1%81%d0%b8%d0%bc%d0%be%d1%81%d1%82%d1%8c-%d1%81%d1%81%d1%8b%d0%bb%d0%be%d0%ba-%d0%bd%d0%b0-%d0%ba%d0%bb%d0%b0%d1%81%d1%81%d1%8b-%d0%b2-python/1545622#1545622