Why Is Piping Output Of Subprocess So Unreliable With Python?

(Windows) I wrote some Python code that calls the program SoX (subprocess module), which outputs the progress on STDERR, if you specify it to do so. I want to get the percentage st

Solution 1:

As per this closely related thread: Unbuffered read from process using subprocess in Python

process = subprocess.Popen('sox_call_dummy.bat', 
                stderr = subprocess.PIPE, bufsize=0)
while True:
    line = process.stderr.readline()
    if not line: 
        break
    print line

Since you aren't reading stdout, I don't think you need a pipe for it.

If you want to try reading char by char as in your original example, try adding a flush each time:

sys.stdout.write(char)
sys.stdout.flush()

Flushing the stdout every time you write is the manual equivalent of disabling buffering for the python process: python.exe -u <script> or setting the env variable PYTHONUNBUFFERED=1