It might be better to use communicate
:
from subprocess import Popen, PIPE, STDOUT
p = Popen(['myapp'], stdout=PIPE, stdin=PIPE, stderr=PIPE)
stdout_data = p.communicate(input="data_to_write")[0]
“Better”, because of this warning:
Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.