Implementing this is actually likely to require non-trivial restructuring of the runpy internals. contextlib.ExitStack may prove useful in making it easier to programmatically select amongst different context managers.

The __mp_main__ helpers in multiprocessing should also be able to take advantage of this once it is available, and it may prove useful in finally providing -m analogues for pdb etc that play nice when an exception occurs (see issue 9325).