Serhiy: "I think the solution can be simpler. PR 3033 uses the global import lock for guarding modification of the _module_locks dictionary." I tried exactly that, but it wasn't enough. But your PR is correct because it also checks the current value of _module_locks[name] using the callback parameter. I disliked my own PR, so I'm happy that someone else wrote a simpler fix! (I already abandonned my PR.)