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 ?

(I think the title you meant was 
"popen spawned process may not 
write to stderr under windows")

The child is dying with IOError: 
[Errno 22] Invalid argument
at the sys.stderr.flush() call.

Neither the docs for os.popen nor 
the Linux man page for popen(3) 
say that stderr is redirected, so 
one would expect the handle to be 
inherited; the IOError looks like 
a bug.
Try using os.popen4 or 
popen2.popen4 or -the recommended 
choice- the subprocess module. 
Using the latter, this is the 
modified parent.py:

"""
import subprocess
cmd = 'python child.py'
p = subprocess.Popen(cmd, 
stdout=subprocess.PIPE)
for line in p.stdout:
  print ">>>", line,
print p.wait()
"""

and this is the output, as 
expected:

"""
2:stderr
>>> 1:stdout
>>> 3:stdout
0
"""

Note the 2:stderr line lacking the 
>>>, because it was printed 
directly by the child process onto 
the stderr handle inherited from 
its parent.

the popen call does not redirect stderr.
If you do something like 2>null (windows) or 2>/dev/null (*nix) it will
_never_ get printed.
If you want to have stderr & stdout getting in via popen and thus stdout,
under *nix and windows you would do that:

command 2>&1

It is not popen to blame.
See this for reference:
http://netbsd.gw.com/cgi-bin/man-cgi?popen++NetBSD-current

pythonmeister: I never expected stderr to be redirected, just *all
stdout* to be captured. But...

gagenellina: you are completely right about the failure. Still, this
issue happened with a real world application written in C, and
redirecting manually stderr to :NUL: solved it unexpectedly. I did not
expect spawned process behaviour to differ when its stderr is being
redirected.

We've discussed this at the PyCon sprints, and here's the concensus:

os.popen inherits the parents stderr, and on Windows there is not an
existing valid stderr by default.  So the parent should, to be
compatible with the Windows environment, create stderr or use popen2.

However, popen is deprecated.  The best solution would be to use
subprocess module which is defined to do the right thing in this case. 
popen is not.

Because popen is deprecated, we are going to leave this behavior and
documentation as it is.

Did you want to close this, Sean?