Indeed, further discussion showed that what they were after was something more along the lines of sarge.Capture: http://sarge.readthedocs.io/en/latest/overview.html#main-features

That is, the ability to start the subprocess running in the background, and access the output line-by-line, but with the precise mechanics of how that works being a hidden implementation detail (just as concurrent.futures hides the details of the inter-process communication for function calls).