I'm trying to understand the proposed solution, "have _imp.is_frozen() check the module in sys.modules." Does that mean it would do a dict lookup first? Maybe you should do that in the caller instead? importlib/_bootstrap.py calls it a few times, but I'm not convinced that all call sites can be called with "os.path" -- do you know which path was taken when this failed?

I worry about the complexity of the importlib bootstrapping mechanism. Grepping through the source for _bootstrap and _bootstrap_external has not given me any understanding of how this works. Do you know if there are docs for this? Or do we just need to ask Brett?