Keep a subprocess alive and keep giving it commands? Python

If I spawn a new subprocess in python with a given command (let's say I start the python interpreter with the python command), how can I send new data to the process (via STDIN)?

Use the standard subprocess module. You use subprocess.Popen() to start the process, and it will run in the background (i.e. at the same time as your Python program). When you call Popen(), you probably want to set the stdin, stdout and stderr parameters to subprocess.PIPE. Then you can use the stdin, stdout and stderr fields on the returned object to write and read data.

Untested example code:

from subprocess import Popen, PIPE

# Run "cat", which is a simple Linux program that prints it's input.
process = Popen(['/bin/cat'], stdin=PIPE, stdout=PIPE)
process.stdin.write(b'Hello\n')
process.stdin.flush()
print(repr(process.stdout.readline())) # Should print 'Hello\n'
process.stdin.write(b'World\n')
process.stdin.flush()  
print(repr(process.stdout.readline())) # Should print 'World\n'

# "cat" will exit when you close stdin.  (Not all programs do this!)
process.stdin.close()
print('Waiting for cat to exit')
process.wait()
print('cat finished with return code %d' % process.returncode)

Don't.

If you want to send commands to a subprocess, create a pty and then fork the subprocess with one end of the pty attached to its STDIN.

Here is a snippet from some of my code:

RNULL = open('/dev/null', 'r')
WNULL = open('/dev/null', 'w')

master, slave = pty.openpty()
print parsedCmd
self.subp = Popen(parsedCmd, shell=False, stdin=RNULL,
                      stdout=WNULL, stderr=slave)

In this code, the pty is attached to stderr because it receives error messages rather than sending commands, but the principle is the same.