alternating stderr with stdout with subprocess.Popen

gistfile1.txt

# ideally we would have output like:
$ python process.py
this is an stderr line

this is an stdout line

this is an stderr line

this is an stdout line

# but we end up having output like:
$ python process.py
this is an stdout line

this is an stdout line

this is an stderr line

this is an stderr line

mix_out.py

import sys

sys.stderr.write('this is an stderr line\n')
sys.stdout.write('this is an stdout line\n')
sys.stderr.write('this is an stderr line\n')
sys.stdout.write('this is an stdout line\n')

process.py

import subprocess
import sys

cmd = ['python', 'mix_out.py']

process = subprocess.Popen(
    cmd,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
    close_fds=True,
)

if process.stdout:
    while True:
        out = process.stdout.readline()
        if out == '' and process.poll() is not None:
            break
        if out != '':
            print out
            sys.stdout.flush()

if process.stderr:
    while True:
        err = process.stderr.readline()
        if err == '' and process.poll() is not None:
            break
        if err != '':
            print err
            sys.stderr.flush()

returncode = process.wait()
if returncode != 0:
        print "command returned non-zero exit status: %s" % returncode