Python global variables not available in function - python-2.7

Question

I am using the following code to read sensor data, process it and publish some feedback to a different topic, /scans_freespace.
#!/usr/bin/env python
import rospy
from rospy.core import is_shutdown
from sensor_msgs.msg import LaserScan
from sweep_bot.msg import Space
from sweep_bot.msg import SpaceArray
class FreeSpace :
def __init__(self) :
rospy.loginfo('starting')
self.scans = ()
self.regions = {}
self.publisher = self.subscriber = ''
def callbackScans(self, data) :
self.scans = data.ranges
self.regions = {
'starboard_aft' : self.scans[0:135],
'starboard_abeam_aft' : self.scans[136:271],
'starboard_abeam_bow' : self.scans[272:407],
'starboard_bow' : self.scans[408:543],
'port_aft' : self.scans[949:1084],
'port_abeam_aft' : self.scans[814:949],
'port_abeam_bow' : self.scans[679:814],
'port_bow' : self.scans[544:679],
}
def publish(self) :
self.getSomeData('port_bow')
def getSomeData(self, region) :
rospy.loginfo("Checking region: %s", region)
rospy.loginfo("Checking scans: %s", self.scans)
return self.scans[region]
if __name__ == '__main__' :
rospy.init_node('space_identifier')
rate = rospy.Rate(10)
spacer = FreeSpace()
subscriber = rospy.Subscriber("scans", LaserScan, spacer.callbackScans)
publisher = rospy.Publisher("scans_freespace", SpaceArray, queue_size=10)
while not rospy.is_shutdown() :
spacer.publish()
rate.sleep()
As the code shows, I am calling the getSomeData() function from my publish() function passing a single string argument. getSomeData() receives the argument as expected but the global variables which are populated from the callbackScan() function are empty.
The output from the script is as follow:
[INFO] [1629830370.246796, 0.000000]: starting
[INFO] [1629830370.253719, 0.000000]: Checking region: port_bow
[INFO] [1629830370.256151, 0.000000]: Checking scans: ()
Traceback (most recent call last):
File "/home/sisko/catkin_ws/src/sweepbot/Sweeper/sweep_bot/src/freespace.py", line 49, in <module>
spacer.publish()
File "/home/sisko/catkin_ws/src/sweepbot/Sweeper/sweep_bot/src/freespace.py", line 31, in publish
self.getSomeData('port_bow')
File "/home/sisko/catkin_ws/src/sweepbot/Sweeper/sweep_bot/src/freespace.py", line 37, in getSomeData
return self.scans[region]
TypeError: tuple indices must be integers, not str
Ironically, the code did work until I attempted to update my output.
What am I missing?

Answer 1

There are a few things wrong here. However, your error is because you do not properly initialize your message fields. This in turn causes issue when publish() is called before a LaserScan message is received. As well, ranges is a std_msg/Float32[] so even if it was assigned correctly in the callback you'd have another error when trying to index a float list with a string. It is a little unclear as to what you want to do, but looking at the key names getSomeData() should look something more like this:
def getSomeData(self, region) :
rospy.loginfo("Checking region: %s", region)
rospy.loginfo("Checking scans: %s", self.scans)
return self.regions[region] if region in self.regions else None
Another point to make is if you're hoping to publish the results of this function out via the publish() function, you need to actually define a publisher inside your class with the correct type.