On Tue, Feb 1, 2011 at 12:30 AM, John O'Hagan <m...@johnohagan.com> wrote: > I'm starting a server process as a subprocess. Startup is slow and > unpredictable (around 3-10 sec), so I'm reading from its stdout until I get a > line that tells me it's ready before proceeding, in simplified form: > > import subprocess > proc = subprocess.Popen(['server', 'args'], stdout=subprocess.PIPE) > while proc.stdout.readline() != "Ready.\n": > pass > > Now I can start communicating with the server, but I eventually realised that > as I'm no longer reading stdout, the pipe buffer will fill up with output from > the server and before long it blocks and the server stops working. > > I can't keep reading because that will block - there won't be any more output > until I send some input, and I don't want it in any case. > > To try to fix this I added: > > proc.stdout = os.path.devnull > > which has the effect of stopping the server from failing, but I'm not > convinced > it's doing what I think it is. If I replace devnull in the above line with a > real file, it stays empty although I know there is more output, which makes me > think it hasn't really worked.
Indeed. proc.stdout is a file, whereas os.devnull is merely a path string; the assignment is nonsensical type-wise. > Simply closing stdout also seems to stop the crashes, but doesn't that mean > it's still being written to, but the writes are just silently failing? In Based on some quick experimentation, yes, more or less. > either case I'm wary of more elusive bugs arising from misdirected stdout. > > Is it possible to re-assign the stdout of a subprocess after it has started? I think that's impossible. (Most of Popen's attributes probably should be read-only properties to clarify that such actions are don't have the intended effect.) > What's the right way to read stdout up to a given line, then > discard the rest? I would think calling Popen.communicate() after you've reached the given line should do the trick. http://docs.python.org/library/subprocess.html#subprocess.Popen.communicate Just ignore its return value. However, that does require sending the input all at once in a single chunk, which it sounds like may not be feasible in your case; if so, I have no idea how to do it cleanly. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
