Discarding STDERR generated during subprocess.popen

[Nobody]

On Mon, 23 Aug 2010 10:38:02 -0700, Leon Derczynski wrote:

> I would like to run an external program, and discard anything written
> to stderr during its execution, capturing only stdout. My code
> currently looks like:
> 
> def blaheta_tag(filename):
>         blaheta_dir = '/home/leon/signal_annotation/parsers/blaheta/'
>         process = subprocess.Popen([blaheta_dir + 'exec/funcTag',
> blaheta_dir + 'data/', filename], cwd=blaheta_dir,
> stdout=subprocess.PIPE)
>         process.wait()
>         return process.communicate()[0]
> 
> This returns stdout, and stderr ends up printing to the console. How
> can I disregard anything sent to stderr such that it doesn't appear on
> the console?

Either:

1. Add "stderr=subprocess.PIPE" to the Popen() call. The communicate()
method will read both stdout and stderr, and you just ignore stderr.

2. Redirect stderr to the null device:

	nul_f = open(os.devnull, 'w')
	process = subprocess.Popen(..., stderr = nul_f)
	nul_f.close()
	return process.communicate()[0]

[os.devnull will be "/dev/null" on Unix, "nul" on Windows.]

BTW: you shouldn't call process.wait() here. The communicate() method will
call the wait() method when it receives EOF.

If you call wait(), and the process tries to write more than a buffer's
worth of output (the exact figure is platform-specific), your script will
deadlock. The child process will block waiting for the script to consume
its output, while the script will block waiting for the child process to
terminate.