I’m sorry, I was on vacation last week and have had to skip some discussions. In this thread there’s a lot of reference to a “set-theoretic” type system for Python (IIUC it is Michael H’s proposal?) but from the one example given (about a function that uses x = x+1; print(x)) I cannot understand the proposal (other than that it might be going back to duck typing, and proposes a technique that AFAIK is used by pytype). Maybe someone can link me to a thread or blog post where this is discussed in more detail?