spawning a process with subprocess

[Diez B. Roggisch]

bhunter schrieb:
> On Nov 26, 1:50 pm, "Diez B. Roggisch" <de... at nospam.web.de> wrote:
>> bhunter schrieb:
>>
>>
>>
>>> Hi,
>>> I've used subprocess with 2.4 several times to execute a process, wait
>>> for it to finish, and then look at its output.  Now I want to spawn
>>> the process separately, later check to see if it's finished, and if it
>>> is look at its output.  I may want to send a signal at some point to
>>> kill the process.  This seems straightforward, but it doesn't seem to
>>> be working.
>>> Here's my test case:
>>> import subprocess, time
>>> cmd = "cat somefile"
>>> thread = subprocess.Popen(args=cmd.split(), shell=True,
>>> stdout=subprocess.PIPE, stdin=subprocess.PIPE,
>>> stderr=subprocess.STDOUT, close_fds=True)
>>> while(1):
>>>       time.sleep(1)
>>>       if(thread.returncode):
>>>          break
>>>       else:
>>>          print thread.returncode
>>> print "returncode = ", thread.returncode
>>> for line in thread.stdout:
>>>    print "stdout:\t",line
>>> This will just print the returncode of None forever until I Ctrl-C it.
>>> Of course, the program works fine if I call thread.communicate(), but
>>> since this waits for the process to finish, that's not what I want.
>>> Any help would be appreciated.
>> I have difficulties understanding what you are after here. To me it
>> looks as if everything works as expected. I mean you periodically check
>> on the liveness of the "thread" - which is what you describe above. All
>> you are missing IMHO is the actual work in this program.
>>
>> So
>>
>> while True:
>>      if do_work():
>>          if thread.returncode:
>>              break
>>      else:
>>          thread.kill()
>>
>> This assumes that your do_work()-method communicates the wish to end the
>> sub-process using it's returnvalue.
>>
>> Diez
> 
> If the subprocess had finished, I expect that the returncode will not
> be None, and the loop would break.  The process hasn't actually
> started.  I know this because while this simple testcase just cats a
> file, the real case submits a simulation job.  This job never starts
> until after I ctrl-c the program.

I don't know what the reason is for that, but I've just today worked 
with code that exactly uses subprocess as advertised - spawning a 
process which runs while the main process occasionally checks inside the 
child's logfiles for certain state changes.

What might be though is that you need to consume the subprocesses stdout 
in your program - because otherwise it will buffer until a certain 
amount (usually 4 or 16k) and then halt.



Diez