Because stat results are cached for performance reasons, it's possible to write to the file system and then try to read/import from it before the modification time for the directory even picks up that there was a change. For this reason there is importlib.invalidate_caches(): https://docs.python.org/3/library/importlib.html#importlib.invalidate_caches . I have opened http://bugs.python.org/issue23422 to track that the docs for importlib.import_module() point out invalidate_caches().