os.system and loggers

[Tim]

On Jan 10, 1:01 pm, Carl Banks <pavlovevide... at gmail.com> wrote:
> On Jan 10, 8:29 am, Tim <jtim.arn... at gmail.com> wrote:
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> > On Jan 7, 11:24 am, Tim <jtim.arn... at gmail.com> wrote:
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> > > hi, I'm using a 3rd-party python program that uses the python logging
> > > facility and also makes calls to os.system. I'm trying to capture its
> > > output to a file.
>
> > > In my own code, I've taken control of the loggers that are setup in
> > > the other program by removing its StreamHandler and replacing with
> > > FileHander. But when it comes to the call to os.system I'm at a loss.
>
> > > I want to capture the stdout from that os.system call in my
> > > FileHandler. I thought this might work, before I call the other
> > > program's class/method:
> > > sys.stdout = getLogger('status').handlers[0].stream
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> > > but no dice. Is there any clean way to get what I want? If not, I
> > > guess I'll override the other method with my own, but it will
> > > basically be a bunch of code copied with os.sytem replaced with
> > > subprocess, using getLogger('status').handlers[0].stream for stdout/
> > > stderr.
>
> > > thanks,
> > > --Tim Arnold
>
> > Replying to my own post....
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> > I think I may have included too much fluff in my original question.
> > The main thing I wonder is whether I can attach a log handler to
> > stdout in such a way that os.system calls will write to that handler
> > instead of the console.
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> No, but you could replace os.system with something that does work.
> (It would replace it globally for all uses, so you may need some logic
> to decide whether to leave the call alone, or to modify it, perhaps by
> inspecting the call stack.)
>
> The simplest thing to do is to append a shell redirection to the
> command (>/your/log/file), so something like this:
>
> _real_os_system = os.system
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> def my_os_system(cmd):
>     if test_log_condition:
>         return _real_os_system(cmd + "2> /my/log/file")
>     return _real_os_system(cmd)
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> os.system = my_os_system
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> That could backfire for any number of reasons so you probably should
> only do this if you know that it works with all the commands it
> issues.
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> The better way might be to call the subprocess module instead, where
> you can dispatch the command with redirection to any stream.  I doubt
> there's a foolproof way to do that given an arbitrary os.system
> command, but the subprocess way is probably safer.
>
> Carl Banks

Thanks Carl. I will use subprocess. I made this little toy example to
prove to myself that subprocess does handle a filehandler stream, so I
include it here for completeness' sake:

import subprocess,logging,shlex

# create the logger, filehandler and get the stream
log = logging.getLogger('mytest')
fh = logging.FileHandler('mytest.log')
log.addHandler(fh)
log.setLevel(logging.INFO)
mylog = logging.getLogger('mytest').handlers[0].stream

# write a test line to the log
log.info('my first line')

# execute the subprocess using the stream as stdout
cmd = 'ls -l'
p = subprocess.Popen(shlex.split(cmd),stdout=mylog)

# if you don't wait(), the output won't be necessarily in
chronological order.
r = p.wait()

# write a last test line to the log
log.info('done %s'% r)

and it works as expected.
thanks,
--Tim Arnold