Very slow update of global variables - python

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Hi

I have made a node which takes input from several different sensors. I want to proces the sensor data and publish a on a new topic.

However when I move the data from the callback functions to my processing algorithm the using global variables, the variables are updated very slowly.

I have a very simplified node of what I have done...

#!/usr/bin/env python

import rospy
from sensor_msgs.msg import LaserScan
from std_msgs.msg import Float64

ranges = []

def callback(data):
     global ranges
     rospy.loginfo(data)
     ranges = data.ranges[200]
def listener():
     rospy.init_node('listener', anonymous=True)
     rospy.Subscriber("scan", LaserScan, callback)      
if __name__ == '__main__':
      while not rospy.is_shutdown():
           listener()
           rospy.Publisher('talk', Float64, queue_size=5).publish(ranges)
           rospy.sleep(0.01)