It seems like pipe might be a suitable choice for your use case. Beware though that under normal circumstance both reading and writing ends expect data to be written or read, respectively. Also make sure you do not get surprised by buffering (nothing comes through because buffers would not get automatically flushed except on an expected boundary, unless set accordingly).
A basic example of how two pipes (they are unidirectional) can be used between two processes:
import os
def child():
"""This function is executed in a child process."""
infile = os.fdopen(r1)
outfile = os.fdopen(w2, 'w', buffering=1)
for line in infile:
if line.rstrip() == 'quit':
break
print(line.upper(), end='', file=outfile)
def parent():
"""This function is executed in a parent process."""
outfile = os.fdopen(w1, 'w', buffering=1)
infile = os.fdopen(r2)
print('Foo', file=outfile)
print(infile.readline(), end='')
print('bar', file=outfile)
print(infile.readline(), end='')
print('quit', file=outfile)
(r1, w1) = os.pipe() # for parent -> child writes
(r2, w2) = os.pipe() # for child -> parent writes
pid = os.fork()
if pid == 0:
child() # child code runs here
elif pid > 0:
parent() # parent code runs here
os.waitpid(pid, 0) # wait for child
else:
raise RuntimeError("This should not have happened.")
Indeed it would be easier and more practical to use subprocess, and you likely want to run another program. The former would require to be told not to close the pipe file descriptors and the latter would require the pipe file descriptors to be inheritable (not have O_CLOEXEC flag set).
Child program:
import os
import sys
infile = os.fdopen(int(sys.argv[1]))
outfile = os.fdopen(int(sys.argv[2]), 'w', buffering=1)
for line in infile:
if line.rstrip() == 'quit':
break
print(line.upper(), end='', file=outfile)
Parent program:
import os
import subprocess
(r1, w1) = os.pipe2(0) # for parent -> child writes
(r2, w2) = os.pipe2(0) # for child -> parent writes
child = subprocess.Popen(['./child.py', str(r1), str(w2)], pass_fds=(r1, w2))
outfile = os.fdopen(w1, 'w', buffering=1)
infile = os.fdopen(r2)
print('Foo', file=outfile)
print(infile.readline(), end='')
print('bar', file=outfile)
print(infile.readline(), end='')
print('quit', file=outfile)
child.wait()
If the child program does not need standard input nor standard output, they could be used to get information respectively in and out of the child program. This would even be simpler.
Child program:
import sys
for line in sys.stdin:
if line.rstrip() == 'quit':
break
print(line.upper(), end='', flush=True)
Parent program:
import os
import subprocess
(r1, w1) = os.pipe2(0) # for parent -> child writes
(r2, w2) = os.pipe2(0) # for child -> parent writes
child = subprocess.Popen(['./child.py'], stdin=r1, stdout=w2)
outfile = os.fdopen(w1, 'w', buffering=1)
infile = os.fdopen(r2)
print('Foo', file=outfile)
print(infile.readline(), end='')
print('bar', file=outfile)
print(infile.readline(), end='')
print('quit', file=outfile)
child.wait()
As stated, it is not really Python specific and these are just rough hints on how pipes as one option could be used.
