provide an interface to access generated output more as a stream

> that's a tricky one. for an example i'd need a well known command that generates stdout while running (and not only prints at the end), [...] spma while yum is doing things.

You found your own example! How about a streamed "yum install foo"
that croaks as soon as the string "Error" appears in the output? How
would that look in this API?