How to use readline() with a pipe returned by subprocess.Popen

Question

I'm calling a child process using subprocess.Popen (Python 2.x on a POSIX system). I want to be able to read the output of the child process using Python's readline() file object function. However, the stream available in Popen.stdout does not appear to have a readline() method.

Using the idea from Python readline from pipe on Linux, I tried the following:

p = subprocess.Popen(
    [sys.executable, "child.py"],
    stdout=subprocess.PIPE)
status = os.fdopen(p.stdout.fileno())
while True:
    s = status.readline()
    if not s:
        break
    print s

However, the problem with this method is that both the p.stdout object and the new status object attempt to close the single file descriptor. This eventually results in:

close failed: [Errno 9] Bad file number

Is there a way to create a file object that "wraps" a previously created file-like object?

how do you get `p.stdout` that has `.fileno()` but no `.readline()` method? — jfs, Jan 24 '12 at 04:43
@J.F.Sebastian: That's a good question. I just tried again and `p.stdout.readline()` works fine. It's possible that I had mistakenly tried `p.readline()`, which is sort of embarrassing. — Greg Hewgill, Jan 24 '12 at 05:16

score 2 · Accepted Answer · edited Jan 20 '12 at 17:20

2

The solution is to use os.dup() to create another file descriptor referring to the same pipe:

status = os.fdopen(os.dup(p.stdout.fileno()))

This way, status has its own file descriptor to close.

edited Jan 20 '12 at 17:20

user unknown

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answered Dec 28 '11 at 21:52

Greg Hewgill

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It's amazing how often simply *writing* a question makes the answer obvious. – Greg Hewgill Dec 28 '11 at 21:53

How to use readline() with a pipe returned by subprocess.Popen

1 Answers1