Interact with standard input/output of another process

Question

I have an executable example.exe. This executable's behaviour is as follows:

1.Waits for input from user
2.Performs some operations, based on input
3.goto 1

How can I use subprocess or a similar module to interact with executable?

I wish to run the process, insert input, receive output, then insert additional inputs based on the output received.

icedtrees · Accepted Answer · 2015-03-16 13:50:03Z

3

from subprocess import Popen, PIPE

process = Popen([r'path/to/process', 'arg1', 'arg2', 'arg3'], stdin=PIPE, stdout=PIPE)

to_program = "something to send to the program's stdin"
while process.poll() == None:  # While not terminated
    process.stdin.write(to_program)

    from_program = process.stdout.readline()  # Modify as needed to read custom amount of output
    if from_program == "something":  # send something new based on stdout
       to_program = "new thing to send to program"
    else:
       to_program = "other new thing to send to program"

print("Process exited with code {}".format(process.poll()))

edited Mar 16, 2015 at 13:50

answered Mar 15, 2015 at 12:10

icedtrees

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5 Comments

Farseer Over a year ago

After first call stdout, stderr = process.communicate(to_program) process terminates... (all additional inputs concidered to be empty string)

Farseer Over a year ago

The communicate() method reads all of the output and waits for child process to exit before returning.

icedtrees Over a year ago

Sorry, should have tested more thoroughly. Try the new example.

jfs Over a year ago

1- in general, you don't know how much should you read and there are other issues with the interactive subprocess usage 2- there is no need to call process.poll() here and it is not enough to call process.poll() -- the process may exit after .poll() call but before the .write() call (unrelated: use is None instead of == None). You should handle IOError errors (EPIPE, EINVAL) raised by .write() instead. If from_program is empty then it means EOF and you could exit the loop.

jfs Over a year ago

3- minor: close the pipes and call process.wait() at the end.

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