How to get topic data to process then publish

edit

Hi all, I'll start off with saying I'm very new to ROS and only just figuring out python too. I've followed all the tutorials and scoured forums but can't find a solution to my issue. I have a ROS package with node 1 publishing message data to a topic, then node 2 subscribes to the topic, processes the data then publishes it again. However I'm not receiving actual values from my topic through the subscriber callback even though they are being published to the topic(confirmed with rostopic echo).

Node 2-

#!/usr/bin/env python

import rospy
import numpy as np
from AR_week4_test.srv import compute_cubic_traj
from AR_week4_test.msg import cubic_traj_param
from AR_week4_test.msg import cubic_traj_coeffs


param = None
coeffs = None

def Computecubics(param):
                pub_coeffs = cubic_traj_coeffs()
                pub_param = cubic_traj_param()

                m_list1 =pub_coeffs
                x = np.array(m_list1)


                m_list2 = [[1,(pub_param.t0),(pub_param.t0**2),(pub_param.t0**3)],
                        [0, 1, (2*pub_param.t0), (3*pub_param.t0**2)],
                        [1, (pub_param.tf), (pub_param.tf**2), (pub_param.tf**3)],
                        [0, 1, (2*pub_param.tf), (3*pub_param.tf**2)]]
                a = np.array(m_list2)
                print(a)
                m_list3 = [[pub_param.p0],
                        [pub_param.v0],
                        [pub_param.pf],
                        [pub_param.vf]]
                b = np.array(m_list3)

                inv_a = np.linalg.inv(a)

                x = np.linalg.inv(A).dot(b)

                coeffs.publish(x)

                rate.sleep()
    if __name__ == '__main__':
        try:

                rospy.init_node('cubic_traj_listener', anonymous=True)

                rate = rospy.Rate(0.05) #20 seconds

                rospy.wait_for_service('computing_cubics')

                rospy.ServiceProxy('computing_cubics', compute_cubic_traj)

                rospy.Subscriber("cubic_traj_params", cubic_traj_param, Computecubics, queue_size=10)

                param = rospy.Publisher("cubic_traj_newparams", cubic_traj_param, queue_size=10)

                rate.sleep()

                rospy.spin()

        except rospy.ROSInterruptException:
                pass

Node 1 -

#!/usr/bin/env python

import rospy
import numpy as np
import random as ra
from std_msgs.msg import Float64MultiArray
from std_msgs.msg import Float32
from AR_week4_test.msg import cubic_traj_param
pos = None
class pointsgen():
        def __init__(self):
                pub = rospy.Publisher('cubic_traj_params', cubic_traj_param, queue_size=10)
                rospy.init_node('points_generator', anonymous=True)

                global pos
                while not rospy.is_shutdown():  
                        pos = cubic_traj_param()
                        pos.p0 = ra.uniform(10,-10)
                        pos.pf = ra.uniform(10,-10)
                        pos.v0 = ra.uniform(10,-10)
                        pos.vf = ra.uniform(10,-10)
                        pos.t0 = 0
                        dt = ra.uniform(5,10)
                        pos.tf =pos.t0+dt

                        print(pos)
                        pub.publish(pos)
                        rospy.loginfo(pos)
                        rate = rospy.Rate(0.05)  # Publish at 0.05hz or 20 times every second
                        rate.sleep()


if  __name__ == "__main__" :
        try:
                ne = pointsgen()
        except rospy.ROSInterruptException: pass

Currently the error I get is a linAlg error due to the matrix m_list2 being filled with zero values