since the time this was filed, subprocess has evolved a lot and third party options for child process have appeared as well as modern things like:

https://docs.python.org/3/library/asyncio-subprocess.html (stdlib)

https://trio.readthedocs.io/en/stable/reference-io.html?highlight=subprocess#options-for-starting-subprocesses (trio, easier async framework to use than asyncio)

plumbing stdin/stdout/stderr data to python file-like objects from the stdlib subprocess APIs themselves is better left to third party solutions building on top of the subprocess module today.