ValueError while looping spin_once() in a separate thread from a custom pyqtgraph widget

edit

I found a solution! My mistake here was trying to use a thread-based solution when I could instead just use multiple processes and inter-process communication provided by QProcess. If anyone has a simple PyQt GUI they want to communicate with ROS, this is a fairly simple (maybe trivial?) solution that allows you to run both rclpy.spin and a PyQt app at the same time, though in different processes.

My code is made up of two scripts: plotter.py and subscriber.py, where plotter.py is the pyqtgraph widget and subscriber.py is the ROS node. To launch both programs, run plotter.py and it will create a new process via QProcess that will launch subscriber.py. The node enters a rclpy.spin loop and outputs a copy of the current message reading to its standard output, only visible by its parent process plotter.py.

I hacked this together very quickly and am fairly new to this so please let me know if there are any obvious improvements to be made. I am unsure whether standard output is the best method for IPC in this case, but it's fast enough for my application.

Here is my code for the ROS node, in subscriber.py

'''
This is a ROS2 subscriber that subscribes to the /cmd_vel topic, and then
"re-publishes" that information to standard output, which is readable by QProcess.
'''

import rclpy
import sys
from rclpy.node import Node
from geometry_msgs.msg import Twist

class QtSubscriber(Node):
    def __init__(self):
        super().__init__('sub_to_qt')

        #init subscription
        self.subs = self.create_subscription(Twist,'cmd_vel',self.callback,10)
        self.subs

    def callback(self,msg):
        # get data from the Twist message
        self.x  = msg.linear.x
        self.y  = msg.linear.y
        self.th = msg.angular.z

        #output
        # NOTE: these messages will be parsed such that the number on the
        # right-hand side of the '=' is read, and if there is no '=', the 
        # whole message is read. So: either make sure everything to the 
        # right of '=' is numeric or make the whole string numeric.
        sys.stdout.write(f"x  = {self.x}\n")
        sys.stdout.write(f"y  = {self.y}\n")
        sys.stdout.write(f"th = {self.th}\n")
        sys.stdout.flush()

def main(args=None):
    rclpy.init(args=args)
    node = QtSubscriber()

    rclpy.spin(node)

    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Here is my code for the pyqtgraph widget, in plotter.py

'''
This script aims to create a pyqtgraph app which also acts as a ros node. 
Timing will be run by the Qt event loop, which should be run at a faster 
rate than ros publishes data at to prevent data loss.
'''
import sys
import pyqtgraph as pg

from PyQt5 import QtWidgets, QtCore
from PyQt5.QtCore import QProcess

from IPython import embed


class MainWindow(pg.GraphicsLayoutWidget):
    '''
    This is a pyqtgraph widget that plots a circular dot in xy space.
    '''
    def __init__(self,parent=None, show=True, size=None, title=None, **kargs):
        super().__init__(parent, show, size, title, **kargs)

        #Initialize ROS subscriber process
        self.p = None
        self ...