Subprocess with and without shell

[George Sakkis]

I'm trying to figure out why Popen captures the stderr of a specific
command when it runs through the shell but not without it. IOW:

cmd = [my_exe, arg1, arg2, ..., argN]
if 1: # this captures both stdout and stderr as expected
    pipe = Popen(' '.join(cmd), shell=True, stderr=PIPE, stdout=PIPE)
else: # this captures only stdout
    pipe = Popen(cmd, shell=False, stderr=PIPE, stdout=PIPE)

# this prints the empty string if not run through the shell
print "stderr:", pipe.stderr.read()
# this prints correctly in both cases
print "stdout:", pipe.stdout.read()

Any hints ?

George