Hi,
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:
Fake\r\nData\r\n
as seen through hexdump -C:
> hexdump -C output.txt
00000000 46 61 6b 65 0d 0a 44 61 74 61 0d 0a |Fake..Data..|
0000000c
Now if I do a simple subprocess.Popen and set the stdout to subprocess.PIPE,
then do p.stdout.read() I get the correct output of
Fake\nData\n
When do the Popen attached to a pty I end up with
Fake\n\nData\n\n
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 a lot of hours of fiddling with
this and googling to no avail.
Thanks,
Jonathan
#!/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__()
self.outputPipe()
self.outputPty()
def outputPipe(self):
p1 = subprocess.Popen(
("/bin/cat", "output.txt"),
stdout=subprocess.PIPE,
universal_newlines=True
)
print "1: %r" % p1.stdout.read()
def outputPty(self):
outMaster, outSlave = pty.openpty()
fcntl.fcntl(outMaster, fcntl.F_SETFL, os.O_NONBLOCK)
p2 = subprocess.Popen(
("/bin/cat", "output.txt"),
stdout=outSlave,
universal_newlines=True
)
with os.fdopen(outMaster, 'rU') as pty_stdout:
while True:
try:
rfds, _, _ = select.select([pty_stdout], [], [], 0.1)
break
except select.error:
continue
for fd in rfds:
buf = pty_stdout.read()
print "2: %r" % buf
if __name__ == "__main__":
t = TestRead()
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