Is not a valid resource graph name and lookupTransform argument target_frame in tf2 frame_ids cannot start with a '/ error?

Course Support TF ROS 101
1

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#! /usr/bin/env python

import sys
import rospy
import tf
import time
import math
from geometry_msgs.msg import Twist

def shutdown_callback():
    global ctr_c
    print("Program is shutting down")
    cmd=Twist()
    cmd.linear.x = 0.0
    cmd.angular.z = 0.0
    turtle_vel.publish(cmd)
    ctrl_c = True

rospy.init_node('TF_listner_turtle')
listner = tf.TransformListener()

if len(sys.argv)<3:
    print("usage: turtle_tf_listener.py follower_model_name model_to_be_followed_name")
else:
    follower=sys.argv[0]
    followed=sys.argv[1]
    turtle_vel = rospy.Publisher(follower+'/cmd',Twist,queue_size=1)
    rate = rospy.Rate(10.0)
    ctr_c=False
    follower_frame="/"+follower
    followed_frame="/"+followed
    rospy.on_shutdown(shutdown_callback)

    while not ctr_c:
        try:
             (trans,rot)=listner.lookupTransform(follower_frame,followed_frame,rospy.Time(0))
        except (tf.LookupException,tf.ConnectivityException,tf.ExtrapolationException):
              continue

        angular = 4 * math.atan2(trans[1], trans[0])
        linear = 0.5 * math.sqrt(trans[0] ** 2 + trans[1] ** 2)
        cmd = Twist()
        cmd.linear.x = linear
        cmd.angular.z = angular
        turtle_vel.publish(cmd)

        rate.sleep()``

This is the error i get
rosrun exercise21 translist.py turtle1 coke_can
the rosdep view is empty: call ‘sudo rosdep init’ and ‘rosdep update’
/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/topics.py:842: UserWarning: ‘/home/user/catkin_ws/src/exercise21/src/translist.py/cmd’ is not a legal ROS graph resource name. This may cause problems with other ROS tools
super(Publisher, self).init(name, data_class, Registration.PUB)
Warning: Invalid argument “/home/user/catkin_ws/src/exercise21/src/translist.py” passed to canTransform argument target_frame in tf2 frame_ids cannot start with a ‘/’ like:
at line 134 in /tmp/binarydeb/ros-kinetic-tf2-0.5.20/src/buffer_core.cpp
Traceback (most recent call last):
File “/home/user/catkin_ws/src/exercise21/src/translist.py”, line 36, in
(trans,rot)=listner.lookupTransform(follower_frame,followed_frame,rospy.Time(0))
File “/opt/ros/kinetic/lib/python2.7/dist-packages/tf/listener.py”, line 104, in lookupTransform
msg = self._buffer.lookup_transform(strip_leading_slash(target_frame), strip_leading_slash(source_frame), time)
File “/opt/ros/kinetic/lib/python2.7/dist-packages/tf2_ros/buffer.py”, line 87, in lookup_transform
return self.lookup_transform_core(target_frame, source_frame, time)
tf2.InvalidArgumentException: Invalid argument “/home/user/catkin_ws/src/exercise21/src/translist.py” passed to lookupTransform argument target_frame in tf2 frame_ids cannot start with a ‘/’ like:

2

Okay got it , it was a bunch of typos.
First for invalid resource name - I gave wrong topic name to publisher /cmd instead of /cmd_Vel
Last error was due to passing invalid robot names used argv[0] and argv[1] , but instead should be argv[1] and argv[2] , apparently on checking sysargv documentation one will find that sysargv[0] is name of python script , not the name of first robot.

I am still keeping this post here hoping to save time for anyone who made silly mistakes like me.