Alexander, please read the earlier comments on the issue: we're deliberately *not* doing that. Such functionality is more advanced, and more appropriate for an API in the subprocess module. Anyone interested in exploring that option further should create a separate issue.
This API is about making a *particular* common use case genuinely trivial. Generalising it *isn't* desirable, as any such generalisations will be sufficiently complex that people are better off just rolling their own solution from first principles.