subprocess communication, exec()

**Jeff McNeil** · Nov 11 '08, 07:45 PM

Re: subprocess communication, exec()

On Nov 11, 1:23 pm, "Chuckk Hubbard" <badmuthahubb.. .@gmail.com>
wrote:

If I run 'python -i subprocessclien t.py' I expect to see the nice
level of it go up 2, and the nice level of the subprocess go up 1.
But all I see is the nice level of the client change. What am I doing
wrong?
>
subprocessserve r.py:
----------------------------
#!/usr/bin/python2.5
>
import os
import sys
>
while True:
next_line = sys.stdin.readl ine()
if not next_line:
break
exec(next_line)
# sys.stdout.writ e(output)
# sys.stdout.writ e(next_line)
# sys.stdout.flus h()
----------------------------
>
subprocessclien t.py:
----------------------------
#!/usr/bin/python2.5
>
import subprocess, os
>
server = subprocess.Pope n(('python2.5', 'subprocessserv er.py'),
shell=True, stdin=subproces s.PIPE, stdout=subproce ss.PIPE,
stderr=subproce ss.PIPE)
>
os.nice(2)
>
server.stdin.wr ite('''os.nice( 1)''')
----------------------------
>
Thanks.
-Chuckk
>
--http://www.badmuthahub bard.com

Looks like you're dropping an error by redirecting the child process'
standard error into a pipe.

First off, remove the stderr=subproce ss.PIPE from
subprocessclien t.py. When you do so, you'll start seeing an error
message whenever your code runs:

[jeff@marvin ~]$ python client.py
None
[jeff@marvin ~]$ Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined

That doesn't make a lot of sense at first, until you consider the
following:

[jeff@marvin ~]$ echo 'os.nice(1)' | python
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined

The Python interpreter is trying to execute the data you write to the
stdin of the child process. Changing the Popen line to use the string
form rather than the sequence form remedies the problem, as does
changing 'string=True' to 'string=False.'

The reason? Because when you set shell=True, additional arguments in
args sequence are passed as additional arguments to the shell itself,
not to the command.

From subprocess.py:

if isinstance(args , types.StringTyp es):
args = [args]
else:
args = list(args)

if shell:
args = ["/bin/sh", "-c"] + args

So, in your attempt, you're effectively doing the following:

/bin/sh -c "python2.5" "server.py"

When you want:

/bin/sh -c "python2.5 server.py"

HTH,

Jeff

**Chuckk Hubbard** · Nov 11 '08, 09:15 PM

Re: subprocess communication, exec()

On Tue, Nov 11, 2008 at 9:39 PM, Jeff McNeil <jeff@jmcneil.n etwrote:

On Nov 11, 1:23 pm, "Chuckk Hubbard" <badmuthahubb.. .@gmail.com>
wrote:

>If I run 'python -i subprocessclien t.py' I expect to see the nice
>level of it go up 2, and the nice level of the subprocess go up 1.
>But all I see is the nice level of the client change. What am I doing
>wrong?
>>
>subprocessserv er.py:
>----------------------------
>#!/usr/bin/python2.5
>>
>import os
>import sys
>>
>while True:
> next_line = sys.stdin.readl ine()
> if not next_line:
> break
> exec(next_line)
># sys.stdout.writ e(output)
># sys.stdout.writ e(next_line)
># sys.stdout.flus h()
>----------------------------
>>
>subprocessclie nt.py:
>----------------------------
>#!/usr/bin/python2.5
>>
>import subprocess, os
>>
>server = subprocess.Pope n(('python2.5', 'subprocessserv er.py'),
>shell=True, stdin=subproces s.PIPE, stdout=subproce ss.PIPE,
>stderr=subproc ess.PIPE)
>>
>os.nice(2)
>>
>server.stdin.w rite('''os.nice (1)''')
>----------------------------
>>
>Thanks.
>-Chuckk
>>
>--http://www.badmuthahub bard.com

>
Looks like you're dropping an error by redirecting the child process'
standard error into a pipe.
>
First off, remove the stderr=subproce ss.PIPE from
subprocessclien t.py. When you do so, you'll start seeing an error
message whenever your code runs:
>
[jeff@marvin ~]$ python client.py
None
[jeff@marvin ~]$ Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined
>
That doesn't make a lot of sense at first, until you consider the
following:
>
[jeff@marvin ~]$ echo 'os.nice(1)' | python
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined
>
The Python interpreter is trying to execute the data you write to the
stdin of the child process. Changing the Popen line to use the string
form rather than the sequence form remedies the problem, as does
changing 'string=True' to 'string=False.'
>
The reason? Because when you set shell=True, additional arguments in
args sequence are passed as additional arguments to the shell itself,
not to the command.
>

From subprocess.py:

>
if isinstance(args , types.StringTyp es):
args = [args]
else:
args = list(args)
>
if shell:
args = ["/bin/sh", "-c"] + args
>
So, in your attempt, you're effectively doing the following:
>
/bin/sh -c "python2.5" "server.py"
>
When you want:
>
/bin/sh -c "python2.5 server.py"
>
HTH,
>
Jeff

That helps immensely, Jeff, thank you for explaining all that. I took
out 'shell=True' and it works, and I took out the stdout and stderr
arguments to debug. I am finally able to perform every step in my
plan: start a child process of python running a script that imports
the Csound API; raise the nice level of the parent process; compile
and run a Csound orchestra remotely; and send it notes from the parent
process.
Thanks for your help.
-Chuckk

--

http://www.badmuthahubbard.com

subprocess communication, exec()

subprocess communication, exec()

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