Note that the setpgid creation part is now somewhat redundant with Popen's start_new_session flag (which calls setsid). Also, this should probably be an option, since with that patch every subprocess is in its own process group.

> I was wondering... what if process A runs a subprocess B which runs a
> subprocess C. Is C still considered a children of A and gets killed as
> well?

No.
When setpgid/setsid is called, a new group is created, so process C will be not be part of the same group as B.