Python Popen().stdout.read() hang

问题

I'm trying to get output of another script, using Python's subprocess.Popen like follows

process = Popen(command, stdout=PIPE, shell=True)
exitcode = process.wait()
output = process.stdout.read()   # hangs here

It hangs at the third line, only when I run it as a python script and I cannot reproduce this in the python shell.

The other script prints just a few words and I am assuming that it's not a buffer issue.

Does anyone has idea about what I am doing wrong here?

回答1:

You probably want to use .communicate() rather than .wait() plus .read(). Note the warning about wait() on the subprocess documentation page:

Warning This will deadlock when using stdout=PIPE and/or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use communicate() to avoid that.

http://docs.python.org/2/library/subprocess.html#subprocess.Popen.wait

回答2:

read() waits for EOF before returning.

You can:

wait for the subprocess to die, then read() will return.
use readline() if your output is broken into lines (will still hang if no output lines).
use os.read(F,N) which returns at most N bytes from F, but will still block if the pipe is empty (unless O_NONBLOCK is set on the fd).