why I can't run the script? (Rospy isn't working)

edit

ros_comm version 1.12.14

SUMMARY
========

PARAMETERS
 * /rosdistro: kinetic
 * /rosversion: 1.12.14

NODES

auto-starting new master
process[master]: started with pid [3691]

Error

[ERROR] [1556954434.843733, 13.516000]: bad callback: <function scan_callback at 0x7f1f058a3578>
Traceback (most recent call last):
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/topics.py", line 750, in _invoke_callback
    cb(msg)
  File "/home/gal/catkin_ws/src/Fira_X_ido/scripts/omerKing/maze_explorer.py", line 27, in scan_callback
    min_right,i_right = min( (range_right[i_right],i_right) for i_right in xrange(len(range_right)) )
ValueError: min() arg is an empty sequence

File:

#!/usr/bin/env python

import rospy, time
from geometry_msgs.msg import Twist
from sensor_msgs.msg import LaserScan

def scan_callback(msg):
    global range_front
    global range_right
    global range_left
    global ranges
    global min_front,i_front, min_right,i_right, min_left ,i_left

    ranges = msg.ranges
    # get the range of a few points
    # in front of the robot (between 5 to -5 degrees)
    range_front[:5] = msg.ranges[5:0:-1]  
    range_front[5:] = msg.ranges[-1:-5:-1]
    # to the right (between 300 to 345 degrees)
    range_right = msg.ranges[300:345]
    # to the left (between 15 to 60 degrees)
    range_left = msg.ranges[60:15:-1]
    # get the minimum values of each range 
    # minimum value means the shortest obstacle from the robot
    min_range,i_range = min( (ranges[i_range],i_range) for i_range in xrange(len(ranges)) )
    min_front,i_front = min( (range_front[i_front],i_front) for i_front in xrange(len(range_front)) )
    min_right,i_right = min( (range_right[i_right],i_right) for i_right in xrange(len(range_right)) )
    min_left ,i_left  = min( (range_left [i_left ],i_left ) for i_left  in xrange(len(range_left )) )


# Initialize all variables
range_front = []
range_right = []
range_left  = []
min_front = 0.0
i_front = 0.0
min_right = 0.0
i_right = 0.0
min_left = 0.0
i_left = 0.0

# Create the node
cmd_vel_pub = rospy.Publisher('cmd_vel', Twist, queue_size = 1) # to move the robot
scan_sub = rospy.Subscriber('scan', LaserScan, scan_callback)   # to read the laser scanner
rospy.init_node('maze_explorer')

command = Twist()
command.linear.x = 0.0
command.angular.z = 0.0

rate = rospy.Rate(10)
time.sleep(1) # wait for node to initialize

near_wall = 0 # start with 0, when we get to a wall, change to 1

# Turn the robot right at the start
# to avoid the 'looping wall'
print("Turning...")
command.angular.z = -0.5
command.linear.x = 0.1
cmd_vel_pub.publish(command)
time.sleep(2)

while not rospy.is_shutdown():

    # The algorithm:
    # 1. Robot moves forward to be close to a wall
    # 2. Start following left wall.
    # 3. If too close to the left wall, reverse a bit to get away
    # 4. Otherwise, follow wall by zig-zagging along the wall
    # 5. If front is close to a wall, turn until clear
    while(near_wall == 0 and not rospy.is_shutdown()): #1
        print("Moving towards a wall.")
        if(min_front > 0.2 and min_right > 0.2 and min_left > 0.2):    
            command.angular.z = 0.0    # if nothing near, go forward
            command.linear.x = 0.45 
            print "going straight"
        elif(min_left < 0.2):           # if wall on left, start tracking
            near_wall = 1       
            print "Left wall"            
        else:
            command.angular.z = -0.25   # if not on left, turn right 
            command.linear.x = 0 ...