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?

Answer 1

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.