python subprocess i/o redirection

output

$ ./runner.py

__NO_REDIRECTION_ON_PARENT__
------ executing main thread ----------

----- parent stdout --------
executing script 1
executing script 2

----- parent stderr --------
script2.sh: line 4: la: command not found

retcode: 127

thread done

__PARENT_STDERR_REDIRECTED_TO_STDOUT__
------ executing main thread ----------

----- parent stdout --------
executing script 1
executing script 2
script2.sh: line 4: la: command not found

----- parent stderr --------

retcode: 127

thread done

runner.py

#!/usr/bin/python
import os, sys, subprocess
import threading

def exec_cmd(cmd):
    cmd_proc = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE, bufsize=1)

    timeout = 600 
    thread_res = {}

    def exec_cmd_thread(result):
        print '------ executing main thread ----------'

        out = ""
        err = ""
        print '\n----- parent stdout --------'
        for line in iter(cmd_proc.stdout.readline, ''):
            sys.stdout.write(line)
            out += line

        print '\n----- parent stderr --------'
        for line in iter(cmd_proc.stderr.readline, ''):
            sys.stdout.write(line)
            err += line

        cmd_proc.communicate()
        print "\nretcode: {0}\n".format(cmd_proc.returncode)

    thread = threading.Thread(target=exec_cmd_thread, args=(thread_res,))
    thread.start()
    thread.join(7)

    if thread.is_alive():
        print 'thread alive... killing'
        cmd_proc.terminate()
    else:
        print 'thread done'

def exec_cmd_no_redirect():
    print "\n__NO_REDIRECTION_ON_PARENT__"
    cmd = "/bin/bash <path_to_script_1.sh>"
    exec_cmd(cmd)

def exec_cmd_redirect():
    print "\n__PARENT_STDERR_REDIRECTED_TO_STDOUT__"
    cmd = "/bin/bash <path_to_script_1.sh> 2>&1"
    exec_cmd(cmd)

exec_cmd_no_redirect()
exec_cmd_redirect()

script1.sh

#!/bin/bash -x

echo 'executing script 1'
/bin/bash script2.sh

script2.sh

#!/bin/bash -x

echo 'executing script 2'
la