Python threaded class calls another threaded class (queue help)

Question

I am trying to control a 3-axis printer using an x-box controller. To get inputs from the x-box I have borrowed code from martinohanlon https://github.com/martinohanlon/XboxController/blob/master/XboxController.py I have also created code that reads a text file line by line (G-code) to move the printer.

I would like to be able to use the X-Box controller to select a G-code file and run it, then as the printer is running continue to listen for a cancel button just in case the print goes wrong. The controller is a threaded class, and my readGcode is a threaded class.

The problem I'm having is that when I use the controller to start the readGcode class I cant communicate with the controller until that thread finished.

My temporary solution is to use the controller to select a file then pass that files path to the readGcode class. In the readGcode class it keeps trying to open a file using a try block and fails until the filepath is acceptable. Then it changes a bool which makes it skip further reading until its done.

Code:

import V2_Controller as Controller
import V2_ReadFile as Read
import time
import sys
# file daialogue
import tkinter as tk
from tkinter import filedialog

# when X is selected on the x-box controller 
def X(xValue):
    if not bool(xValue):
        try:
            f=selectfile()
            rf.setfilename(f)
        except:
            print("failed to select file")

def selectfile():
    try:
        root = tk.Tk()  # opens tkinter
        root.withdraw()  # closes the tkinter window
        return filedialog.askopenfilename()
    except Exception:
        print("no file")

# setup xbox controller
xboxCont = Controller.XboxController(controlCallBack, deadzone=30, 
scale=100, invertYAxis=True)
# init the readfile class
rf = Read.Readfile()

# set the custom function for pressing X
xboxCont.setupControlCallback(xboxCont.XboxControls.X, X)

try:
    # start the controller and readfile threads
    xboxCont.start()
    rf.start()

    xboxCont.join()
    rf.join()

    while True:
        time.sleep(1)

# Ctrl C
except KeyboardInterrupt:
    print("User cancelled")

# error
except:
    print("Unexpected error:", sys.exc_info()[0])
    raise

finally:
    # stop the controller
    xboxCont.stop()
    rf.stop()

V2_Readfile

# Main class for reading the script
class Readfile(threading.Thread):

    # supports all variables needed to read a script
    class readfile:
        fileselect = True
        linecount = 0
        currentline = 0
        commands = []

    # setup readfile class
    def __init__(self):
        # setup threading
        threading.Thread.__init__(self)
        # persist values
        self.running = False
        self.reading = False

    def setfilename(self,filename):
        self.filename = filename

    # called by the thread
    def run(self):
        self._start()

    # start reading
    def _start(self):
        self.running = True
        while self.running:
            time.sleep(1)
            if not self.reading:
                try:
                    self.startread()
                except:
                    pass

    def startread(self):
        try:
            with open(self.filename, "r") as f:  # read a local file
                f1 = f.readlines()
                # run through each line and extract the command from each line
                linecount = 0
                line = []
                for x in f1:
                    # read each line into an array
                    line.append(x.split(";")[0])
                    linecount += 1

                # Store the variables for later use
                self.readfile.linecount = linecount
                self.readfile.commands = line
                self.reading = True
        except Exception:
            pass

        i = 0
        while i < self.readfile.linecount and self.reading:
            self.readfile.currentline = i + 1
            self.readline(i)
            i += 1

        # the following stops the code from reading again
        self.reading = False
        self.filename = ""


    def readline(self,line):
        Sort.sortline(self.readfile.commands[line])

    # stops the controller
    def stop(self):
        self.running = False

Answer 1

You could use a syncronization primitive like threading.Event .

To do so, you need to modify your Readfile class like this:

from threading import Event


class Readfile(threading.Thread):

    # setup readfile class
    def __init__(self):
        # setup threading
        threading.Thread.__init__(self)
        # persist values
        self.running = False
        self.reading = False
        self.reading_file = Event()  # Initialize your event here it here

    def setfilename(self,filename):
        self.filename = filename
        self.reading_file.set()  # This awakens the reader

    def _start(self):
        self.running = True
        self.reading_file.wait()  # Thread will be stopped until readfilename is called
        self.startread()

Another syncronization primitive worth exploring is queue.Queue . It could be useful if you want to process more than one filename.

The pattern you describe in your question is called Busy Waiting , and should be avoided when possible.