This script allows to move and draw a square on the ground with the turtlebot in three different ways

move_square.py

import math
import rospy as ros
import sys
import time

from geometry_msgs.msg import Twist, Pose
from nav_msgs.msg import Odometry
from tf.transformations import euler_from_quaternion, quaternion_from_euler


__author__ = "Gabriel Urbain" 
__copyright__ = "Copyright 2018, IDLab, UGent"

__license__ = "MIT" 
__version__ = "1.0" 
__maintainer__ = "Gabriel Urbain"
__email__ = "[email protected]" 
__status__ = "Education" 
__date__ = "October 15th, 2018"


class SquareMove(object):
    """
    This class is an abstract class to control a square trajectory on the turtleBot.
    It mainly declare and subscribe to ROS topics in an elegant way.
    """

    def __init__(self):

        # Declare ROS subscribers and publishers
        self.node_name = "square_move"
        self.odom_sub_name = "/odom"
        self.vel_pub_name = "/cmd_vel"
        self.vel_pub = None
        self.odometry_sub = None

        # ROS params
        self.pub_rate = 0.1
        self.queue_size = 2

        # Variables containing the sensor information that can be used in the main program
        self.odom_pose = None

    def start_ros(self):

        # Create a ROS node with a name for our program
        ros.init_node(self.node_name, log_level=ros.INFO)

        # Define a callback to stop the robot when we interrupt the program (CTRL-C)
        ros.on_shutdown(self.stop_robot)

        # Create the Subscribers and Publishers
        self.odometry_sub = ros.Subscriber(self.odom_sub_name, Odometry, callback=self.__odom_ros_sub, queue_size=self.queue_size)
        self.vel_pub = ros.Publisher(self.vel_pub_name, Twist, queue_size=self.queue_size)

    def stop_robot(self):

        # Get the initial time
        self.t_init = time.time()

        # We publish for a second to be sure the robot receive the message
        while time.time() - self.t_init < 1 and not ros.is_shutdown():
            
            self.vel_ros_pub(Twist())
            time.sleep(self.pub_rate)

        sys.exit("The process has been interrupted by the user!")

    def move(self):
        """ To be surcharged in the inheriting class"""

        while not ros.is_shutdown():
            time.sleep(1)

    def __odom_ros_sub(self, msg):

        self.odom_pose = msg.pose.pose

    def vel_ros_pub(self, msg):

        self.vel_pub.publish(msg)


class SquareMoveVel(SquareMove):
    """
    This class implements a open-loop square trajectory based on velocity control. HOWTO:
     - Start the roscore (on the computer or the robot, depending on your configuration)
            $ roscore
     - Start the sensors on the turtlebot:
            $ roslaunch turtlebot3_bringup turtlebot3_robot.launch
     - Start this node on your computer:
            $ python move_square vel
    """

    def __init__(self):
        
        super(SquareMoveVel, self).__init__()

    def go_forward(self, duration, speed):

        # Get the initial time
        self.t_init = time.time()

        # Set the velocity forward and wait (do it in a while loop to keep publishing the velocity)
        while time.time() - self.t_init < duration and not ros.is_shutdown():

            msg = Twist()
            msg.linear.x = speed
            msg.angular.z = 0
            self.vel_ros_pub(msg)
            time.sleep(self.pub_rate)

    def turn(self, duration, ang_speed):

         # Get the initial time
        self.t_init = time.time()

        # Set the velocity forward and wait 2 sec (do it in a while loop to keep publishing the velocity)
        while time.time() - self.t_init < duration and not ros.is_shutdown():

            msg = Twist()
            msg.linear.x = 0
            msg.angular.z = ang_speed
            self.vel_ros_pub(msg)
            time.sleep(self.pub_rate)

    def move(self):

        self.go_forward(2, 0.5)
        self.turn(3.5, 0.5)
        self.go_forward(2, 0.5)
        self.turn(3.5, 0.5)
        self.go_forward(2, 0.5)
        self.turn(3.5, 0.5)
        self.go_forward(2, 0.5)
        self.stop_robot()


class SquareMoveOdom(SquareMove):
    """
    This class implements a semi closed-loop square trajectory based on relative position control,
    where only odometry is used. HOWTO:
     - Start the roscore (on the computer or the robot, depending on your configuration)
            $ roscore
     - Start the sensors on the turtlebot:
            $ roslaunch turtlebot3_bringup turtlebot3_robot.launch
     - Start this node on your computer:
            $ python move_square odom
    """

    def __init__(self):


        super(SquareMoveOdom, self).__init__()

        self.pub_rate = 0.1

    def get_z_rotation(self, orientation):

        (roll, pitch, yaw) = euler_from_quaternion([orientation.x, orientation.y, orientation.z, orientation.w])
        print roll, pitch, yaw
        return yaw
        
    def move_of(self, d, speed=0.5):

        x_init = self.odom_pose.position.x
        y_init = self.odom_pose.position.y

        # Set the velocity forward until distance is reached
        while math.sqrt((self.odom_pose.position.x - x_init)**2 + \
             (self.odom_pose.position.y - y_init)**2) < d and not ros.is_shutdown():

            sys.stdout.write("\r [MOVE] The robot has moved of {:.2f}".format(math.sqrt((self.odom_pose.position.x - x_init)**2 + \
            (self.odom_pose.position.y - y_init)**2)) +  "m over " + str(d) + "m")
            sys.stdout.flush()

            msg = Twist()
            msg.linear.x = speed
            msg.angular.z = 0
            self.vel_ros_pub(msg)
            time.sleep(self.pub_rate)

        sys.stdout.write("\n")

    def turn_of(self, a, ang_speed=0.4):

        # Convert the orientation quaternion message to Euler angles
        a_init = self.get_z_rotation(self.odom_pose.orientation)
        print a_init

        # Set the angular velocity forward until angle is reached
        while (self.get_z_rotation(self.odom_pose.orientation) - a_init) < a and not ros.is_shutdown():

            # sys.stdout.write("\r [TURN] The robot has turned of {:.2f}".format(self.get_z_rotation(self.odom_pose.orientation) - \
            #     a_init) + "rad over {:.2f}".format(a) + "rad")
            # sys.stdout.flush()
            # print (self.get_z_rotation(self.odom_pose.orientation) - a_init)

            msg = Twist()
            msg.angular.z = ang_speed
            msg.linear.x = 0
            self.vel_ros_pub(msg)
            time.sleep(self.pub_rate)

        sys.stdout.write("\n")

    def move(self):

        # Wait that our python program has received its first messages
        while self.odom_pose is None and not ros.is_shutdown():
            time.sleep(0.1)

        # Implement main instructions
        # self.move_of(0.5)
        self.turn_of(math.pi/4)
        self.move_of(0.5)
        self.turn_of(math.pi/4)
        self.move_of(0.5)
        self.turn_of(math.pi/4)
        self.move_of(0.5)
        self.stop_robot()


class SquareMoveOdomIMU(SquareMoveOdom):
    """
    This class implements a semi closed-loop square trajectory based on relative position control,
    where only odometry is used. HOWTO:
     - Start the roscore (on the computer or the robot, depending on your configuration)
            $ roscore
     - Start the sensors on the turtlebot:
            $ roslaunch turtlebot3_bringup turtlebot3_robot.launch
     - Start a node for sensors fusion the turtleot (IMU + Odometry are now merged in a new odometry message):
            $ roslaunch robot_pose_ekf robot_pose_ekf.launch imu_used:=true odom_used:=true vo_used:=false
     - Start this node on your computer:
            $ python move_square odom
    """

    def __init__(self):


        # The functions are the same as in the previous example except for the odometry name that is now filtered
        # in a, Extended Kalman Filter (EKF) with the IMU values thanks to the node robot_pose_ekf
        super(SquareMoveOdomIMU, self).__init__()
        self.odom_sub_name = "/robot_pose_ekf/odom_combined"


if __name__ == '__main__':

    # Choose the example you need to run in the command line
    if len(sys.argv) > 1:

        if sys.argv[1] == "vel":
            r = SquareMoveVel()

        elif sys.argv[1] == "odom":
            r = SquareMoveOdom()

        elif sys.argv[1] == "odom_imu":
            r = SquareMoveOdomIMU()

        else:
            sys.exit(-1)

    else:
        sys.exit(-1)

    # Listen and Publish to ROS + execute moving instruction
    r.start_ros()
    r.move()

When I am running this code in python its returns error " An exception has occurred, use %tb to see the full traceback "

When I am running this code in python its returns error " An exception has occurred, use %tb to see the full traceback "

If you look at the code, you will see you get a sys.exit(-1) if you don't provide args to the command line... So, depending on your python stack, this can be expected behaviour. Please, try to read through the code and google it first.
Also, this code is old, may not be up-to-date with more recent versions or ROS and I don't guarantee it will run correctly.