New line conversion with Popen attached to a pty

jfharden at jfharden at
Thu Jun 20 15:28:37 CEST 2013


Sorry if this appears twice, I sent it to the mailing list earlier and the mail seems to have been swallowed by the black hole of email vagaries.

We have a class which executes external processes in a controlled environment and does "things" specified by the client program with each line of output. To do this we have been attaching stdout from the subprocess.Popen to a pseudo terminal (pty) made with pty.openempty and opened with os.fdopen. I noticed that we kept getting a bunch of extra new line characters.

This is all using python 2.6.4 in a centos6 environment.

After some investigation I realised we needed to use universal_newline support so I enabled it for the Popen and specified the mode in the fdopen to be rU. Things still seemed to be coming out wrong so I wrote up a test program boiling it down to the simplest cases (which is at the end of this message). The output I was testing was this:

as seen through hexdump -C:

> hexdump -C output.txt 
00000000  46 61 6b 65 0d 0a 44 61  74 61 0d 0a              |Fake..Data..|

Now if I do a simple subprocess.Popen and set the stdout to subprocess.PIPE, then do I get the correct output of


When do the Popen attached to a pty I end up with


Does anyone know why the newline conversion would be incorrect, and what I could do to fix it? In fact if anyone even has any pointers to where this might be going wrong I'd be very helpful, I've done hours of fiddling with this and googling to no avail.

One liner to generate the test data:

python -c 'f = open("output.txt", "w"); f.write("Fake\r\nData\r\n"); f.close()'

Test script:

#!/usr/bin/env python2.6.4
import os
import pty
import subprocess
import select
import fcntl

class TestRead(object):

    def __init__(self):
        super(TestRead, self).__init__()

    def outputPipe(self):
        p1 = subprocess.Popen(
            ("/bin/cat", "output.txt"),
        print "1: %r" %

    def outputPty(self):
        outMaster, outSlave = pty.openpty()
        fcntl.fcntl(outMaster, fcntl.F_SETFL, os.O_NONBLOCK)

        p2 = subprocess.Popen(
            ("/bin/cat", "output.txt"),

        with os.fdopen(outMaster, 'rU') as pty_stdout:
            while True:
                    rfds, _, _ =[pty_stdout], [], [], 0.1)
                except select.error:

            for fd in rfds:
                buf =
                print "2: %r" % buf

if __name__ == "__main__":
    t = TestRead()



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