So in this case that would be, a FooBarWidget is not a subclass of FooWidget but maybe still a subclass of AbstractWidget above that. It contains a thread and config as its state variables. That thread instantiates a FooWidget with the saved config, and runs it, and finally closes its queue.

The problem still occurs, because you have to define what it means to close a FooBarWidget and I don't think python Thread has a “throw an exception into this thread” method. Just setting the should_exit property, has the same problem as the post! The thread might still be initing the object and any attempt to tweak across threads could sometimes tweak before init is complete because init does I/O. But once you are there, this is just a tweak of the FooWidget code. FooWidget could respond to a lock, a semaphore, a queue of requests, any number of things, to be told to shut down.

In fact, Python has a nice built-in module called asyncio, which implements tasks, and tasks can be canceled and other such things like that, probably you just wanted to move the foowidget code into a task. (See @jackdied's great talk “Stop Writing Classes” for more on this general approach. It's not about asyncio specifically, rather it's just about how the moment we start throwing classes into our code, we start to think about things that are not just solving the problem in front of us, and solving the problem in front of us could be done with just simple structures from the standard library.)