[Baypiggies] Subprocess: Common problem/common pattern
Minesh B. Amin
mamin at mbasciences.com
Sat Oct 2 19:23:40 CEST 2010
Hi Glen,
The are three general ways:
+ commands.getstatusoutput(...)
when you:
* do not mind waiting for the command to finish
* are interested in the stdout+stderr at the end (when the
command is finished)
+ pexpect < http://www.noah.org/wiki/Pexpect#Overview >
when you:
* want to interact with the launched command; e.g. enter login,
password, answer Y/N
Thanks to pexpect, most of these interactions may be automated;
i.e.
the python script may take over responsibility of processing
"login", "password" requests.
Any interaction not automated may be passed on to the "human".
+ The solution "Capturing the Output and Error Streams from a
Unix Shell Command" from the book "Python Cookbook"
when you:
* want to simply consume the stderr/stdout of the launched command
in real-time
Hope this helps.
Cheers!
Minesh
On Sat, 2010-10-02 at 09:44 -0700, Glen Jarvis wrote:
> I've now seen a common problem come up several times. And, I imagine
> there is a common solution to this problem that I don't know.
>
>
>
> About a year ago, I wrote an automatic script that would automatically
> do an 'svn export' of certain branches of a tree depending upon what
> menu option the customer chose. When I did the svn export, I used
> subprocess.Popen.
>
>
> The pattern was similar to the following:
>
>
> print """This output is being buffered so I can read the version
> number.
> .... I'm not stuck, just busy exporting files....
>
> """
>
> .....
>
> process = subprocess.Popen(['svn', 'export', repository,
> repo_name], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
>
>
> stdoutdata, stderrdata = process.communicate()
>
>
>
>
> I printed "please wait" and then printed the data given when the
> process was done (stdoutdata). It wasn't ideal but, it was sufficient
> for the time. If I were to have gone for the best fix, I would
> probably have learned the API for subversion to integrate directly
> into python.
>
>
> However, another BayPIGgie is having the same issue. He is using a
> command to start a CD burner from the command line and wants to print
> the output as it is being created from the command line.
>
>
> I can see that to solve this we wouldn't use the communicate()
> convenience function. A few 'hackish' ways that may solve this, but
> I'm looking for the common pattern that is used when other pythonista
> run up against this problem. I also want to ensure that I don't have a
> 'hack' that causes a deadlock only to discover this much later after
> I've implemented the pattern a few times.
>
>
> To help keep the conversation more focused, I've created two tiny test
> programs for a test case:
> 1) A C command line program that we have no control over changing
> within python, and
> 2) A Python program that calls that the c-program (baypiggies_demo):
>
>
> Compile command line so output is same as expected by Python program:
> gcc -pedantic baypiggies_demo.c -o baypiggies_demo
>
>
>
>
> ---- start of c program: File: baypiggies_demo.c ---
> #include <stdio.h>
> #include <unistd.h>
>
>
> int
> main()
> {
> int i = 0;
>
>
> printf("Silly output for monitoring...\n");
> for (i = 0; i <= 10; i++) {
> printf("Counting... %d\n", i);
> sleep(1);
> }
> }
> --- end of c program ---
>
>
>
>
>
>
> --- start of python program to demonstrate. File baypiggies.py ---
> import subprocess
>
>
> print "Just waiting...."
>
>
> process = subprocess.Popen(['./baypiggies_demo'],
> stdout=subprocess.PIPE)
>
>
> stdoutdata, stderrdata = process.communicate()
>
>
> print "Well, NOW I get it.. :( "
> print stdoutdata
> --- end baypiggies.py --
>
>
>
>
>
>
> Has anyone else ran into this before? What's the classic pattern used?
>
>
>
>
> Thanks in advance,
>
>
>
>
> Glen
>
> --
> Whatever you can do or imagine, begin it;
> boldness has beauty, magic, and power in it.
>
> -- Goethe
>
>
> _______________________________________________
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--
MBA Sciences, Inc (www.mbasciences.com)
Tel #: 650-938-4306
Email: mamin at mbasciences.com
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