subprocess and non-blocking IO (again)

Marc Carter mcarter at uk.ibm.com
Mon Oct 10 17:04:23 CEST 2005


I am trying to rewrite a PERL automation which started a "monitoring" 
application on many machines, via RSH, and then multiplexed their 
collective outputs to stdout.

In production there are lots of these subprocesses but here is a 
simplified example what I have so far (python n00b alert!)
- SNIP ---------
import subprocess,select,sys

speakers=[]
lProc=[]

for machine in ['box1','box2','box3']:
     p = subprocess.Popen( ('echo '+machine+';sleep 2;echo goodbye;sleep 
2;echo cruel;sleep 2;echo world'), stdout=subprocess.PIPE, 
stderr=subprocess.STDOUT, stdin=None, universal_newlines=True )
     lProc.append( p )
     speakers.append( p.stdout )

while speakers:
     speaking = select.select( speakers, [], [], 1000 )[0]
     for speaker in speaking:
         speech = speaker.readlines()
         if speech:
             for sentence in speech:
                 print sentence.rstrip('\n')
                 sys.stdout.flush() # sanity check
         else: # EOF
             speakers.remove( speaker )
- SNIP ---------
The problem with the above is that the subprocess buffers all its output 
when used like this and, hence, this automation is not informing me of 
much :)

In PERL, "realtime" feedback was provided by setting the following:
$p->stdout->blocking(0);

How do I achieve this in Python ?

This topic seems to have come up more than once.  I am hoping that 
things have moved on from posts like this:
http://groups.google.com/group/comp.lang.python/browse_thread/thread/5472ce95eb430002/434fa9b471009ab2?q=blocking&rnum=4#434fa9b471009ab2
as I don't really want to have to write all that ugly 
fork/dup/fcntl/exec code to achieve this when high-level libraries like 
"subprocess" really should have corresponding methods.

If it makes anything simpler, I only *need* this on Linux/Unix (Windows 
would be a nice extra though).

thanks for reading,
Marc



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