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PO"3-way" Python subprocess pipeline: send stdout and stderr to two different processes?
 

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    <p>Can the "standard" subprocess pipeline technique (e.g. <a href="http://docs.python.org/2/library/subprocess.html#replacing-shell-pipeline" rel="nofollow">http://docs.python.org/2/library/subprocess.html#replacing-shell-pipeline</a>) be "upgraded" to two pipelines?</p> <pre><code># How about p1 = Popen(["cmd1"], stdout=PIPE, stderr=PIPE) p2 = Popen(["cmd2"], stdin=p1.stdout) p3 = Popen(["cmd3"], stdin=p1.stderr) p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits. p1.stderr.close() #p2.communicate() # or p3.communicate()? </code></pre> <p>OK, it's actually a different use case, but the closest starting point seems to be the pipeline example. By the way, how does p2.communicate() in a "normal" pipeline drive p1? Here's the normal pipeline for reference:</p> <pre><code># From Python docs output=`dmesg | grep hda` # becomes p1 = Popen(["dmesg"], stdout=PIPE) p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE) p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits. output = p2.communicate()[0] </code></pre> <p>I guess I'm ultimately interested in what kind of "process graphs" (or maybe just trees?) can <code>communicate()</code> support, but we'll leave the general case for another day.</p> <p><strong>Update</strong>: Here's the baseline functionality. Without communicate(), create 2 threads reading from p1.stdout and p2.stdout. In the main process, inject input via p1.stdin.write(). The question is whether we can drive a 1-source, 2-sink graph using just communicate()</p>
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    1. COrelated: [How do I write stderr to a file while using “tee” with a pipe?](http://stackoverflow.com/q/692000/4279)
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    2. COThanks... but I'm looking to nail down communicate() behaviour in Python
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    3. COhere's a reverse example: [output from several subprocesses is collected via a single pipe in a single thread](http://stackoverflow.com/a/9745864/4279) i.e., `n-source -> 1-sink`
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