Question

I’m writing a python script that uses subprocess.Popen to execute two programs (from compiled C code) which each produce stdout. The script gets that output and saves it to a file. Because the output is sometimes large enough to overwhelm subprocess.PIPE, causing the script to hang, I send the stdout directly to the log file. I want to have my script write something to the beginning and end of the file, and between the two subprocess.Popen calls. However, when I look at my log file, anything I wrote to the log file from the script is all together at the top of the file, followed by all the executable stdout. How can I interleave my added text to the file?

def run(cmd, logfile):
    p = subprocess.Popen(cmd, shell=True, universal_newlines=True, stdout=logfile)
    return p

def runTest(path, flags, name):
    log = open(name, "w")
    print >> log, "Calling executable A"
    a_ret = run(path + "executable_a_name" + flags, log)
    print >> log, "Calling executable B"
    b_ret = run(path + "executable_b_name" + flags, log)
    print >> log, "More stuff"
    log.close()

The log file has:
Calling executable A
Calling executable B
More stuff
[… stdout from both executables …]

Is there a way I can flush A’s stdout to the log after calling Popen, for example? One more thing that might be relevant: Executable A starts then pends on B, and after B prints stuff and finishes, A then prints more stuff and finishes.

I’m using Python 2.4 on RHE Linux.

Answer 1

You could call .wait() on each Popen object in order to be sure that it’s finished and then call log.flush(). Maybe something like this:

def run(cmd, logfile):
    p = subprocess.Popen(cmd, shell=True, universal_newlines=True, stdout=logfile)
    ret_code = p.wait()
    logfile.flush()
    return ret_code

If you need to interact with the Popen object in your outer function you could move the .wait() call to there instead.