unable to run ros package executable

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Hello, I am trying to run a python script using the rosrun command, but each time I attempt to do this I get this message

Usage: rosrun [--prefix cmd] [--debug] PACKAGE EXECUTABLE [ARGS]
rosrun will locate PACKAGE and try to find
an executable named EXECUTABLE in the PACKAGE tree.
If it finds it, it will run it with ARGS.

The command I use to run my python script is rosrun assignment_subscriber.py

I have tried the following:

$ source devel/setup.bash

chmod +x assignment_subscriber.py

but ros will still not run the script. I am very new to ros so I may be overlooking something very simple as far as configuration, but I still haven't figured it out. Please help. Thanks

assignement_subscriber.py

#!/usr/bin/env python

import rospy
import turtle


def poseCallback(pose_message):

rospy.loginfo(" Postion:%s, %d, %z" % (data.x, data.y, data.theta))
print ("pose callback")
print ('y = %z' %)    
print ('y = %f' %)
print ('yaw = '.%) 

if __name__ == '__main__':
  try:

    rospy.init_node('turtlesim_motion_pose', anonymous=True)        

    rospy.Subscriber("turtle_position", Position, poseCallback)

    rospy.spin()


except rospy.ROSInterruptException:
    rospy.loginfo("node terminated.")