Launch a subprocess in a pseudo terminal (pty), and interact with both the process and its pty.
Sometimes, piping stdin and stdout is not enough. There might be a password prompt that doesn't read from stdin, output that changes when it's going to a pipe rather than a terminal, or curses-style interfaces that rely on a terminal. If you need to automate these things, running the process in a pseudo terminal (pty) is the answer.
Interface:
from ptyprocess import PtyProcessUnicode
p = PtyProcessUnicode.spawn(['python'])
p.read(20)
p.write('6+6\n')
p.read(20)