subprocess.Popen and ordering writes to stdout and stderr

[Mark Hammond]

On 18/12/2009 9:33 AM, Chris Withers wrote:
> exarkun at twistedmatrix.com wrote:
>> libc is probably giving you line buffering when you use os.system
>> (because the child process inherits the parent's stdio, and the
>> parent's stdio is probably a pty, and that's the policy libc implements).
>
> <snip>
>
> Interesting, but do these assertions still hold true when I tell you
> that I'm doing all this on Windows? ;-)

Yep.  You can see similar behaviour from just the cmd-prompt and the 
following script:

--
import sys
for i in range(10):
   print "stdout"
   print >> sys.stderr, "stderr"
--

If you execute it "normally" from a command-prompt, you will see things 
written in the correct order.  If you execute it like 'python foo.py > 
out 2>&1', the order will be mixed up.  If you execute it like 'python 
-u foo.py > out 2>&1' the order is restored.

HTH,

Mark