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Subscribers not making callbacks

asked 2017-08-02 10:24:28 -0500

j3ven7 gravatar image

updated 2017-08-02 11:17:20 -0500

I have a listener function that subscribes to two topics as shown below, but the callbacks (which I confirmed are updating) don't seem to update some variables in the listener. I confirm that the variable (in this case gps_xy) is updated in the gps callback, but for some reason, the value is never modified in listener(). I thought since python is multi-threaded that spinning should not be necessary but I could be wrong. Any ideas? Also this code is executable, I call the listener in a standard if __name__ == '__main__':

def listener():

    pub = rospy.Publisher('kalman_pub', Float32MultiArray, queue_size=10) 
    rospy.init_node('kalman', anonymous=True)
    rospy.Subscriber("bike_state", Float32MultiArray, bike_state)
    rospy.Subscriber("gps", Float32MultiArray, gps)
    rate = rospy.Rate(5)
    #Run until the nodes are shutdown ( run OR was killed)
    while not rospy.is_shutdown():
        dim = [MultiArrayDimension('data', 1, 4)]
        layout = MultiArrayLayout(dim, 0)
        # The Kalman filter wants the GPS data in matrix form
        #Build matrix from gps x,y coordinates and bike velocity and yaw
        gps_matrix = np.matrix(gps_xy + bike_yv + time_step)
        #save gps state values for later plotting
        C = np.matrix([[1, 0, 0, 0], [0, 1, 0, 0], [0, 0, 1, 0], [0, 0, 0, 1]])
        #If we have 
        if len(gps_data) == 1:
            P_initial = np.matrix([[1, 0, 0, 0], [0, 1, 0, 0], [0, 0, 1, 0], [0, 0, 0, 1]])
            x_pos = gps_matrix[:,0]
            y_pos = gps_matrix[:,1]
            yaw = gps_matrix[:,2]
            v = gps_matrix[:,3]
            v_0 = v.item(0)
            yaw_0 = yaw.item(0)
            x_dot_0 = v_0 * np.cos(yaw_0)
            y_dot_0 = v_0 * np.sin(yaw_0)
            s_initial = np.matrix([[x_pos.item(0)], [y_pos.item(0)], [x_dot_0], [y_dot_0]])
            #output matrix - returns a tuple - first entry - kalman state (x,y,x',y')
            #                             - second entry - prediction error (p)
            output_matrix = kalman_real_time.kalman_no_loop(gps_matrix, C, s_initial, P_initial)
            output_matrix = kalman_real_time.kalman_no_loop(gps_matrix, C, 
                                                 kalman_state_matrix, p_state_matrix) 
        kalman_state_matrix = output_matrix[0] 
        p_state_matrix = output_matrix[1]                                           
        #Change output_matrix to a standard array for publishing
        kalman_state = output_matrix[0].flatten().tolist()[0]
        p_state = output_matrix[1].flatten()
        #save predicted state values for later plotting
        #pub.publish(layout, kalman_state)
        pub.publish(layout, kalman_state)
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1 Answer

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answered 2017-08-02 11:27:03 -0500

Almost certainly the issue has to do with scoping in Python. In your callbacks you likely have some line looking approximately like gps_xy = This is creating a new variable local to the callback that goes out of scope as soon as the callback is done running. Here are several ways you could fix this:

  • Force the variables that you want updated to be global variables with the global keyword in your callbacks.
  • Make the variables, callbacks, and pubs/subs part of a class. Then your callback will receive a copy of self as its first argument, and you'll be able to access self.gps_xy
  • Use the callback_args argument in the Subscriber instantiation method to pass additional arguments to your callback. In this case, it would be the variables that you would want updated within the callbacks.
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Asked: 2017-08-02 10:24:28 -0500

Seen: 2,242 times

Last updated: Aug 02 '17