I was trying to use subprocess to run multiple processes, and then wait
until one was finished.  I was using poll() to do this and created the
following test case:
#BEGIN
import subprocess,os

procs = [subprocess.Popen(["sleep",str(x)]) for x in range(1,11)]

while len(procs) > 0:
    os.wait()
    print [(p.pid,p.poll()) for p in procs]
    procs = [p for p in procs if p.poll() == None]
#END

I would have expected that as this program was run, it would remove the
processes that finished from the procs list, but instead, they stay in
it and I got the following output:

#Output
[(7426, None), (7427, None), (7428, None), (7429, None), (7430, None),
(7431, None), (7432, None), (7433, None), (7434, None), (7435, None)]
#above line repeats 8 more times
[(7426, None), (7427, None), (7428, None), (7429, None), (7430, None),
(7431, None), (7432, None), (7433, None), (7434, None), (7435, None)]
Traceback (most recent call last):
  File "./test_poll.py", line 9, in <module>
    os.wait()
OSError: [Errno 10] No child processes
#End output

Basically, even for finished processes, poll returns None.

Version of python used:
Python 2.5.1 (r251:54863, Oct 30 2007, 13:45:26) 
[GCC 4.1.2 20070925 (Red Hat 4.1.2-33)] on linux2

Relevant documentation in Library reference manual 17.1.2
poll(  	) ... Returns returncode attribute.
... A None value indicates that the process hasn't terminated yet.