Common.parent is supposed to be one of Common's subclasses, basically, it's parent: Common. But in reality, things are much, much stricter - for example, tmt.steps.execute.internal.ExecuteInternal will never become parent of tmt.base.Test. For vario0us classes in tmt codebase, the list of possible parents is way stricter than Common. Formalizing this situation would give mypy and other linter more knowledge, preventing bugs and helping linters and editors analyzing code.