Hello, guys I am making basic run and stop program and I found a problem. I don't know why when the call back function runs the self.l_scan = scan_data updates like it should. But when the is_obstacle member function runs. The self.l_scan is empty. I have probably some spell mistake, but I tried to look for it like for hour and can find the mistake. The topic /scan is from Turtlebot and it runs like it should. I tried via echo.

#!/usr/bin/env python
import rospy
from sensor_msgs.msg import LaserScan
from geometry_msgs.msg import Twist
import math

class Robot(object):
    def __init__(self):
        # Params
        self.l_scan = LaserScan()
        self.loop_rate = rospy.Rate(10)
        self.velocity = Twist()

        self.pub = rospy.Publisher("/cmd_vel", Twist, queue_size=10)
        self.sub = rospy.Subscriber("/scan", LaserScan, self.callback)

    def callback(self,scan_data):
        self.l_scan = scan_data
        print("readed")

    def is_obstacle(self,distance):
        print(self.l_scan)
        slice_of_array = self.l_scan.ranges[174: 184+1]
        if min(slice_of_array) < distance:
            return True
        else:
            return False

    def stop(self):
        print("stop")    
        self.velocity = Twist()
        self.pub.publish(self.velocity)

    ## move until find obstacle closer than 0.6, laser [-5,+5] degrees
    def move(self):
        self.velocity.linear.x = 1

        while not self.is_obstacle(0.6):
            print("moving")
            self.pub.publish(velocity)
            self.loop_rate.sleep()
        self.stop()

## rotate until find obstacle greater then 3
##def rotate():
##    global l_scan
##    laser_sub = rospy.Subscriber("/scan", LaserScan, scan_callback)
##    vel_pub = rospy.Publisher("/cmd_vel", Twist)

if __name__ == '__main__':
    try:
        rospy.init_node('Turtle_robot_control', anonymous=True)
        my_robot = Robot()
        my_robot.move()

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

Output

header: 
  seq: 0
  stamp: 
    secs: 0
    nsecs:         0
  frame_id: ''
angle_min: 0.0
angle_max: 0.0
angle_increment: 0.0
time_increment: 0.0
scan_time: 0.0
range_min: 0.0
range_max: 0.0
ranges: []
intensities: []
readed
Traceback (most recent call last):
  File "/home/trahery/catkin_ws/src/ros_essentials_cpp/src/topic04_perception02_laser/robot_control.py", line 54, in <module>
    print(my_robot.is_obstacle(0.2))
  File "/home/trahery/catkin_ws/src/ros_essentials_cpp/src/topic04_perception02_laser/robot_control.py", line 24, in is_obstacle
    if min(slice_of_array) < distance:
ValueError: min() arg is an empty sequence