Concerned about race conditions while accessing SQLite3 database connection that's accessed in thread invoked by Pynput listener inside a QThread

Question

I\'m writing a Windows application with Pyside2. Due to the nature of how I\'m using multithreading, I\'m having to interact with the same Sqlite3 database in multiple threads.

Answer 1

First of all a QThread is not a Qt thread, that is, it is not a new type of thread, QThread is a class that manages the native threads of each platform. so the thread that handles QThread has the same characteristics of threading.Thread.

On the other hand the goal of using threads in a GUI is not to block the main thread called GUI thread, in your pynput it already has its thread so there would be no problems. The other task that is blocking is that of the OCR, so we must execute it in a new thread. The task of the database is not expensive, so it is not necessary to create a thread.

keymonitor.py

from pynput import keyboard
import time
from PySide2 import QtCore

class KeyMonitor(QtCore.QObject):
    letterPressed = QtCore.Signal(str)

    def __init__(self, parent=None):
        super().__init__(parent)
        self.listener = keyboard.Listener(on_release = self.on_release)

    def on_release(self,key):
        if type(key) == keyboard._win32.KeyCode:
            self.letterPressed.emit(key.char.lower())

    def stop_monitoring(self):
        self.listener.stop()

    def start_monitoring(self):
        self.listener.start()

imageprocess.py

import uuid
import pytesseract

from PIL import ImageGrab

from PySide2 import QtCore

class ProcessWorker(QtCore.QObject):
    processSignal = QtCore.Signal(str, str)

    def doProcess(self):
        screenshot = ImageGrab.grab()
        screenshot_path =  QtCore.QDir.current().absoluteFilePath(uuid.uuid4().hex+".jpg")
        screenshot.save(screenshot_path )
        print("start ocr")
        ocr_string = pytesseract.image_to_string(screenshot)
        print(ocr_string, screenshot_path)
        self.processSignal.emit(ocr_string, screenshot_path)
        self.thread().quit()

main.py

from keymonitor import KeyMonitor
from imageprocess import ProcessWorker
from PySide2 import QtCore, QtWidgets

import sqlite3

class Widget(QtWidgets.QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.last_letter = ""
        self.current_letter = ""

        lay = QtWidgets.QVBoxLayout(self)
        button = QtWidgets.QPushButton("Start")
        button.clicked.connect(self.onClicked)
        lay.addWidget(button)

        self.keymonitor = KeyMonitor()
        self.keymonitor.letterPressed.connect(self.onLetterPressed)

        self.db_connection = sqlite3.connect("testdababase.db")
        self.cursor = self.db_connection.cursor()
        self.cursor.execute("CREATE TABLE IF NOT EXISTS testdb (OCRstring text, filepath text)")
        self.threads = []

    def onClicked(self):
        self.keymonitor.start_monitoring()

    def onLetterPressed(self, letter):
        if self.last_letter:
            if self.current_letter:
                self.last_letter = self.current_letter
            self.current_letter = letter
        else:
            self.last_letter = letter

        if self.last_letter == "j" and self.current_letter == "k":
            print("j+k")
            self.start_processing()

    def start_processing(self):
        thread = QtCore.QThread()
        self.worker = ProcessWorker()
        self.worker.processSignal.connect(self.onProcessSignal)
        self.worker.moveToThread(thread)
        thread.started.connect(self.worker.doProcess)
        thread.finished.connect(self.worker.deleteLater)
        thread.finished.connect(lambda th=thread: self.threads.remove(th))
        thread.start()
        self.threads.append(thread)

    def onProcessSignal(self, ocr, path):
        print(ocr, path)
        self.cursor.execute("INSERT INTO testdb (OCRstring, filepath) VALUES (?,?)",(ocr, path))
        self.db_connection.commit()

if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = Widget()
    w.show()
    sys.exit(app.exec_())