Python subprocess PIPE that works on Python 2 but not on Python 3

Question

I'm trying to control a software MIDI synthesizer Timidity++ with subprocess in Python. The way I'm doing it is to start a process of Timidity++ in its Ncurses UI, and then pass commands to the STDIN of the subprocess. This way I can control the playback, for example adjust keys, in real-time.

The problem is that the approach seems to work on Python 2 (specifically Python 2.7.13), but not on Python 3 (specifically Python 3.5.3). What's the reason for that? I'm testing this on Raspbian 9.

My code is like the following:

from subprocess import *
import time
a = Popen(["timidity", "-in", "MIDI_sample.mid"], stdin=PIPE, stdout=PIPE)
time.sleep(4)
a.stdin.write(b"+")
time.sleep(4)
a.stdin.write(b"s")
time.sleep(4)
a.stdin.write(b"s")
time.sleep(4)
a.stdin.write(b"q")

This seems to work well on Python 2, ie you can hear the playback, pause and unpause, and key adjustments. On Python 3 it just plays through the MIDI file until the program terminates.

Answer 1

As Davis pointed out in the above comment, flushing a.stdin solved the problem. So the following code works for both Python 2 and 3:

from subprocess import *
import time
a = Popen(["timidity", "-in", "MIDI_sample.mid"], stdin=PIPE, stdout=PIPE)
time.sleep(4)
a.stdin.write(b"+")
a.stdin.flush()
time.sleep(4)
a.stdin.write(b"s")
a.stdin.flush()
time.sleep(4)
a.stdin.write(b"s")
a.stdin.flush()
time.sleep(4)
a.stdin.write(b"q")
a.stdin.flush()