Odd Problems with subprocess.py

After a fantastic presentation on subprocess at the SoCal Python Interest Group last June, I bought into it completely. That recently translated into making the twill unit tests work properly on Windows (I switched from fork() to subprocess) and also building some of the functional tests for Cartwheel. For the Cartwheel tests, I'm using it to do database setup and teardown by running the psql command in various ways.

I spent part of my food-poisoning convalescence yesterday getting the Cartwheel functional tests running in a buildbot buildslave, and I ran into the darndest problem. subprocess was causing the tests to hang, because the stdout buffer was filling up and in turn causing psql output to hang. Any command that had only a little bit of output worked fine, while any command that output more than a 1kb or two of text, failed. If I sent stdout anywhere other than a pipe, it worked fine -- but I needed the output!

After an hour or so, I gave up trying to figure out how to configure subprocess properly and simply wrote a function that converted the stdout and stderr handles to non-blocking and then polled them continuously for new data. (Yes, I probably could use select, but it was failing for reasons I didn't understand.) Quite an ugly hack, but I do need the output for consistency checking!

Anyway, my code is below, for those who like to nitpick. It's ugly.

The really strange thing for me is that this hanging problem didn't occur on my development machine; only my buildslave (a Linux VM box running much the same software) had problems. The meta-lesson here is (once again) that continuous integration really does help you find the darndest things, including mysterious configuration/environment related problems that you never would have thought of looking for.


def _poll_subprocess(cmd):
    # make stdout/stderr non-blocking
    stdout_fileno = cmd.stdout.fileno()
    stderr_fileno = cmd.stderr.fileno()

    flags = fcntl.fcntl(stdout_fileno, fcntl.F_GETFL)
    fcntl.fcntl(stdout_fileno, fcntl.F_SETFL, flags| os.O_NONBLOCK)

    flags = fcntl.fcntl(stderr_fileno, fcntl.F_GETFL)
    fcntl.fcntl(stderr_fileno, fcntl.F_SETFL, flags| os.O_NONBLOCK)

    # wait for the process to end, sucking in stuff until it does end
    stdout = stderr = ""
    while cmd.poll() is None:
            stdout += cmd.stdout.read()
        except IOError:

            stderr = cmd.stderr.read()
        except IOError:

    # get the last output
        stdout += cmd.stdout.read()
    except IOError:

        stderr += cmd.stderr.read()
    except IOError:

    return (stdout, stderr, cmd.poll())

Legacy Comments

Posted by Scott Lamb on 2007-02-09 at 15:52.

"A kilobyte" or two is PIPE_BUF bytes (4096 on my system). This is a
classic deadlock situation - process A is blocking on adding bytes to
the A->B pipe, but process B allow it to proceed (by consuming the
bytes already there)  until process A does something else.    It's
described in the Python documentation <a
control.html">here</a>, though unfortunately in a spot most people
won't see anymore since popen2 is obsoleted by subprocess.    Anyway,
I think subprocess's "communicate" function does the same thing as
your loop. Another option is to send stdout and stderr to a

Posted by Micha Kwiatkowski on 2007-02-09 at 16:24.

I had the same problem with Cheesecake just a few days ago. I solved
it by redirecting output to a temporary file, so it will be written
there, not blocking the output producer process.    Changeset with fix
for Cheesecake: <a href="http://pycheesecake.org/changeset/175">http:/

Posted by bear on 2007-02-09 at 18:36.

I ran into this issue last year when exploring subprocess myself.  I
determined that if I tried to wait() on the process the buffer quickly
filled up (especially since I was redirecting stderr to stdout to get
the output intermixed.)    I solved it by letting subprocess handle
the polling:    import subprocess  p = subprocess.Popen(['foo',
stdout=subprocess.PIPE, stderr=subprocess.STDOUT)    out =
p.stdout.readlines()    p.wait()    with the above I no longer was
getting "mystery" hangs.  This even caused me to do a rare blog post
about it:    <a href="http://code-bear.com/bearlog/2006/03/02">http

Posted by Titus Brown on 2007-02-09 at 20:18.

Doing the same thing with 'read()' instead of readlines() didn't work
for me.  Maybe readlines does something different?

Posted by Scott Lamb on 2007-02-09 at 21:43.

read() vs. readlines() shouldn't matter. bear's code fragment works
because stdout and stderr go to the same stream. If they were separate
and it read from one, the subprocess could get blocked on the other.

Posted by Titus Brown on 2007-02-09 at 22:30.

Ahh, good tip.  I'll try that, then.    thanks,  --titus

Posted by Josiah Carlson on 2007-02-12 at 20:37.

Or you can use the async subprocess subclass available on the Python
cookbook.  It supports both *nix and Windows.    <a href="http://aspn.

Posted by Greg Wilson on 2007-02-13 at 16:21.

I spent several hours messing around with subprocess last summer,
trying to find an easy way to explain it to people without strong
backgrounds in CS (the intended audience for Software Carpentry).  I
eventually gave up in frustration: there are just too many options and

Posted by Titus Brown on 2007-03-24 at 12:29.

Scott, bear -- the 'communicate' function does exactly what I need.
Thanks, Scott!    --titus

Posted by Noah Gift on 2007-06-14 at 08:23.

Is there anything subprocess did not take over from os?

Posted by Titus Brown on 2007-06-15 at 20:02.

from 'os.system'?  not much, as far as I can tell.  However, 'system'
is apparently a POSIX standard so it's being left in.    --titus

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