New submission from Clovis Fabricio <nosklo+pyt...@gmail.com>: Suppose I want to simulate the following shell pipe using the subprocess module:
`grep -v not | cut -c 1-10` The documentation example here http://docs.python.org/library/subprocess.html#replacing-shell-pipeline Implies that I want to run this: grep_process = Popen(["grep", "-v", "not"], stdin=PIPE, stdout=PIPE) cut_process = Popen(["cut", "-c", "1-10"], stdin=p1.stdout, stdout=PIPE) grep_process.stdin.write('Hello World\n') grep_process.stdin.close() result = cut_process.stdout.read() # blocks forever here assert result == "Hello Worl\n" When grep_prcoess starts, two file descriptors are created, one for each end of the pipe. Lets call those `grep-stdin-w` and `grep-stdout-r`. When I run cut_process, `grep-stdout-r` gets passed as cut_process sdtin. Since `close_fds=False` by default, The effect of that is that cut_process also inherits `grep-stdin-w`. So `grep` can't die even if I explicty run `grep_process.stdin.close()` because `grep-stdin-w` stdin is still inside cut_process (`cut` ignores the extra open fd). Passing `close_fds=True` to the second Popen call makes the code work perfectly. `close_fds=True` should be the default on unix systems, because it closes files descriptors that have nothing to do with the process, leaving stdin, stdout and stderr working, which is the intended behaviour of most code. ---------- components: Library (Lib) messages: 96032 nosy: nosklo severity: normal status: open title: when piping output between subprocesses some fd is left open blocking forever type: behavior versions: Python 2.7, Python 3.2 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue7448> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
