Let child.py be:
"""
import sys

sys.stdout.write('1:stdout\n')
sys.stdout.flush()
sys.stderr.write('2:stderr\n')
sys.stderr.flush()
sys.stdout.write('3:stdout\n')
sys.stdout.flush()
"""

and parent.py:
"""
import os

cmd = 'python child.py'
for l in os.popen(cmd):
    print l,
"""

Then running it:
"""
>python parent.py
1:stdout

>python
Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit
(Intel)] on win32
"""

I would have expected at least:
"""
1:stdout
3:stdout
"""
to be output, which actually happens if the stderr is nullified like
"python child.py 2>nul" in parent.py call.

Am I wrong ?