Run part of the fabric script sequentially (failed)

fabfile.py

from fabric.api import env
# from fabric.api import serial
from fabric.api import task
from fabric.api import run
from fabric.task_utils import state

env.hosts = ['addv3.redbeacon-inc.com', 'addv4.redbeacon-inc.com']
env.parallel = True


# @serial < does not work on function
def go_sleep_serial():
    env.parallel = False  # Does not work
    state.env['parallel'] = False  # does not work
    run('sleep 10')
    env.parallel = True
    state.env['parallel'] = True


def go_sleep_parallel():
    run('sleep 10')


@task
# @serial < running task should be assigned as serial
# it seems like execution strategy is decided start of task call
# and cannot change later.
def start_serial():
    go_sleep_serial()


@task
def start_parallel():
    go_sleep_parallel()