[Tutor] Interacting with stderr

[Cameron Simpson]

Before we proceed, two requests:

Please do not top post. Post below, and trim the irrelevant content, so things 
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keeping it out of my main inbox.

Returning to your subprocess questions...

On 27Aug2014 20:28, Crush <crushed26 at gmail.com> wrote:
>As far as sending the contents of stderr to a tmp file...hmmm, i
>will try that. My only concern is, many of these systems run live
>demo feeds for many days...10+. Im afraid the file will fill up
>very quickly and present problems.

Ok. Then a file may not be entirely suitable to your task.

>Stderr=PIPE causes the terminal to hang. I can not wait for the process to terminate, i need to read stderr as it is being fed data.. Would fifos be of any use here?

First off, stderr=PIPE _is_ a FIFO; it constructs a pipe between the subprocess 
and your python program.

However, if you need to read the stream as it occurs then you should not use 
"call()", which inherently waits for the command to finish.

Instead, use the Popen constructor (also part of the subprocess module).

Crude and incomplete and untested example:

   from subprocess import Popen, PIPE

   P = Popen("avconv ... lots of arguments...", shell=True, stderr=PIPE)

   for line in P.stderr:
       ... examine the line from stderr ...

   # ok, we have read all of stderr now
   xit = P.wait()
   if xit != 0:
       ... command was unsuccessful, complain, maybe abort ...

Cheers,
Cameron Simpson <cs at zip.com.au>

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