Jeff: This makes it somewhat easier to handle input and output as strings instead of streams. Most of the functionality was already there, but this makes it more broadly useful. It doesn't especially address your other points, but I'm not aiming to completely overhaul subprocess.

> for string commands, shell should always be true. for list/Tupperware commands, shell should be false

I wondered why this is not the case before, but on Windows a subprocess is actually launched by a string, not a list. And on POSIX, a string without shell=True is interpreted like a one-element list, so you can do e.g. Popen('ls') instead of Popen(['ls']). Changing that would probably break backwards compatibility in unexpected ways.