If it possible to launch a launch file from python?

edit

Final update: The solution so far is modified from here with help from ufr3c_tjc. (Thank you loudly in my heart) So the original codes from the link above is erroneous and won't work. The main part is remained:

class open_launch_file(): 
    def __init__(self):
        rospy.init_node('tester', anonymous=True)
        rospy.on_shutdown(self.shutdown)

        uuid = roslaunch.rlutil.get_or_generate_uuid(None, False)
        roslaunch.configure_logging(uuid)
        launch = roslaunch.parent.ROSLaunchParent(uuid,["/path/to/your/launch/file"])
        launch.start()

    def shutdown(self):
        rospy.loginfo("Stopping the robot...")
        destroy() #my own cleaning stuff function, you dont have to have it. but if there is anything you need to do before close the ros node, do it here.
        rospy.sleep(1)

===============================================================================

I have a .launch file that basically just open the webcam. And I would like to call it from my python code when needed. I searched online and found something like this: I have test other normal commands like "ls -l", "echo Hello world". And they are working fine. However, it would not work with the launch file

import subprocess
p = subprocess.Popen(["roslaunch ~/path/to/cam.launch "],stdout=subprocess.PIPE,stderr = subprocess.PIPE,shell=True)
print p.communicate()

the error message is:

'/bin/sh: 1: roslaunch: not found\n

I am using python 2.7

update:

the output of printenv | grep PY is:

INSIDE_CAJA_PYTHON=
PYTHONPATH=/opt/ros/kinetic/lib/python2.7/dist-packages

When using the API example provide by ROS, there were errors that confuse me. I have set the environment correctly I believe. When I rosrun tester_pkg tester.py , it gives error NameError: name 'rospy' is not defined

So I tried sudo python tester.py directly, it gives ImportError: No module named roslaunch

Thank you all for the replying. I am new to ROS and really need some helps.

update codes: tester.py

#!/usr/bin/env python
import roslaunch
import rospy

rospy.init_node('tester', anonymous=True)
rospy.on_shutdown(self.shutdown)

uuid = roslaunch.rlutil.get_or_generate_uuid(None, False)
roslaunch.configure_logging(uuid)
launch = roslaunch.parent.ROSLaunchParent(uuid,["/home/yue/catkin_ws/src/webcam_car/launch/cam.launch"])

launch.start()

launch.shutdown()