Catching external program exceptions

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Wed Jan 2 01:13:01 CET 2008


En Tue, 01 Jan 2008 20:57:44 -0200, Marty <martyb1 at earthlink.net> escribi�:

> I need to catch exceptions thrown by programs started by the os.system  
> function,
> as indicated by a non-zero return code (e.g. the mount utility).  For

Notice that these are two different things. You can wait until the  
external program ends, and get its return code; and you can sometimes read  
a message from its stderr. But none of these things by itself throws an  
exception in Python.

> example,
> if I get the following results in a bash shell:
>
> $mount test
> mount: can't find /home/marty/test in /etc/fstab or /etc/mtab
>
> then I want to catch the same exception from the corresponding  
> os.system() call,
> i.e. "os.system('mount test')", but it doesn't work as expected:

Perhaps it doesn't work as *you* expected, but it does work as specified.  
 From the os.system description at  
http://docs.python.org/lib/os-process.html
"On Unix, the return value is the exit status of the process encoded in  
the format specified for wait(). [...] [That return value] is  
system-dependent."
Later on the same page, describing the wait function, says that the return  
value is "a 16-bit number, whose low byte is the signal number that killed  
the process, and whose high byte is the exit status (if the signal number  
is zero)"
So os.system does NOT raise an exception when the executed program ends  
with a non-zero exit code.

> I get the same results with popon, popen2, popen3, etc.  Apparently  
> these also
> work only when the program does not generate an exception.

An external program cannot generate an exception inside the Python  
program. Only Python itself can do that.

> Is there any way to
> catch the return code. or if not, a workaround?

 From the description above, you could do some math to obtain the exit code  
looking at the os.system return value.
But perhaps it's easier to use subprocess.check_call, see "Convenience  
functions" at http://docs.python.org/lib/module-subprocess.html

-- 
Gabriel Genellina




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