subprocess escaping POpen?!

[Chris Withers]

Hi All,

I have a script that does the following:

from subprocess import Popen,PIPE,STDOUT

def execute(command,cwd):
     return Popen(
         command,
         stderr=STDOUT,
         stdout=PIPE,
         universal_newlines=True,
         cwd=cwd,
         shell=True,
         ).communicate()[0]

captured = execute('svn up .')

Now, if the subversion update requires authentication credentials, it 
manages to write to the console running the above script, *and* read 
input from it too.

This is a bit baffling to me, I thought Popen.communicate() was happily 
hoovering all the output to stdout and stderr into the result returned 
from communicate?

And, indeed, if I change the script instead to do:

import sys
f = open('test.py','w')
f.write('import sys; sys.stderr.write("Hello!\\n")')
f.close()
captured = execute('test.py')

...then the output is indeed captured. So, what is svn doing 
differently? How is it escaping its jail?

Chris