python: nonblocking subprocess, check stdout

Question

Ok so the problem I'm trying to solve is this:

I need to run a program with some flags set, check on its progress and report back to a server. So I need my script to avoid blocking while the program executes, but I also need to be able to read the output. Unfortunately, I don't think any of the methods available from Popen will read the output without blocking. I tried the following, which is a bit hack-y (are we allowed to read and write to the same file from two different objects?)

import time
import subprocess
from subprocess import *
with open("stdout.txt", "wb") as outf:
    with open("stderr.txt", "wb") as errf:
        command = ['Path\To\Program.exe', 'para', 'met', 'ers']
        p = subprocess.Popen(command, stdout=outf, stderr=errf)
        isdone = False
        while not isdone :
            with open("stdout.txt", "rb") as readoutf: #this feels wrong
                for line in readoutf:
                    print(line)
            print("waiting...\r\n")
            if(p.poll() != None) :
                done = True
            time.sleep(1)
        output = p.communicate()[0]    
        print(output)

Unfortunately, Popen doesn't seem to write to my file until after the command terminates.

Does anyone know of a way to do this? I'm not dedicated to using python, but I do need to send POST requests to a server in the same script, so python seemed like an easier choice than, say, shell scripting.

Thanks! Will

Answer 1

Basically you have 3 options:

1. Use threading to read in another thread without blocking the main thread.
2. select on stdout, stderr instead of communicate. This way you can read just when data is available and avoid blocking.
3. Let a library solve this, twisted is a obvious choice.