The following errors are coming up when i run my roscore

AttributeError: 'RosPack' object has no attribute 'get_custom_cache'
[master] killing on exit
Unhandled exception in thread started by
sys.excepthook is missing
lost sys.stderr

i believe my export is working correct what could be the issue? The program it is for is attached also.

#!/usr/bin/python
#========================================
#
# EV3 ROS Driver edited by Theodore Cornelius Smith
# Filename: serial_proj.py
#
#========================================
# Import the serial
import serial
# Import the rospy module
import rospy
# Import ROS Msgs
from std_msgs.msg import Int8
from geometry_msgs.msg import Twist
# Import ev3 driver messages
# Might use generic messages, but for now we need the header data
# to determine network integrity
# Dict for EV3 peripherals (sensors + motors)
p = {}
cs_pub = None
def init_ros_publishers():
    global cs_pub
    cs_pub =rospy.Publisher('arlobot/ping',UInt8MultiArray,queue_size=10)
def publish_ping_sensor():
    global cs_pub
    global startMarker, endMarker
    startMarker = '<'
    endMarker = '>'
    ck = ""
    x = "z" # any value that is not an end- or startMarker
    byteCount = -1 # to allow for the fact that the last increment will be one too many
    # wait for the start character
    while  ord(x) != startMarker: 
     x = ser.read()
    # save data until the end marker is found
     while ord(x) != endMarker:
       if ord(x) != startMarker:
         ck = ck + x 
         byteCount += 1
       x = ser.read()
# Add sensors from Guynays code here
    cs_pub.publish(ck.split('_'))
# print.cs_pub
def subscribe_cmd_vel(data):
    global p
    potential_left = (data.linear.x - data.angular.z)* 255
    if potential_left > 255:
        potential_left = 255
    elif potential_left < -255:
        potential_left = -255
    potential_right = (data.linear.x + data.angular.z) * 255
    if potential_right > 255:
        potential_right = 255
    elif potential_right < -255:
        potential_right = -255
    str = '<' + potential_Left + ', ' + potential_Left + '>'
# print.str
    ser.write(str)
def on_shutdown():
    global cs_pub
    cs_pub.unregister()
serPort = "/dev/ttyACM0"
baudRate = 9600
ser = serial.Serial(serPort, baudRate)
print "Serial port " + serPort + " opened  Baudrate " + str(baudRate)
# Main function
def arlo_driver():
    rospy.init_node('arlo_driver',anonymous=False)
    init_ros_publishers()
    s = rospy.Subscriber("arlobot/cmd_vel",Twist,subscribe_cmd_vel,queue_size=10)
    rate = rospy.Rate(100) # 25Hz
    while not rospy.is_shutdown():  
        if s.get_num_connections() == 0:
            publish_ping_sensor()
        rate.sleep()

if __name__ == '__main__':
    try:
            arlo_driver()
    except rospy.ROSInterruptException:
        pass