Now I have a deeper understanding I think this can be handled separately to Issue 16217.

This patch pulls together my previous two patches, and adds a fix. There are two aspects of my fix:

1. Import the package before calling find_spec() on the __main__ submodule. This means exceptions raised by the initialization code can be differentiated from exceptions from find_spec().

2. Change all the special ImportError exceptions raised inside runpy [and also one raised by InspectLoader.get_code()] to an internal _Error exception known only to runpy. Now I can be sure that all _Error exceptions are not caused by the initialization code, and I can stop catching ImportError, but still catch _Error and suppress the traceback. When runpy is invoked from the documented run_module() or run_path() APIs, _Error is not used, and it still raises ImportError to maintain backwards compatibility.

I think my patch should avoid the main problem in Issue 19771 as well.

Please review my patch. There are so many error possibilities; it is hard to be sure I have got them all right.