python - Proper way to quit/exit a PyQt program

Question

I have a script which has a login screen and if the cancel button is pressed, I want to exit the application altogether. I have tried 3 ways:

1. sys.exit()
2. QApplication.quit()
3. QCoreApplication.instance().quit()

Only number 1 works. The other two makes the dialog box white and it flashes then hangs and I cannot even switch to other applications. My code is below:

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtSql import *
from PyQt5.QtWidgets import *


import csv
import sys
from datetime import datetime, timedelta, time
import os

from ci_co_table import *
from login import *

class Ci_Co(QMainWindow):
    """Check in and check out module"""

    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)

class Login(QDialog):
    """User login """
    def __init__(self):
        QDialog.__init__(self)
        self.ui = Ui_login_form()
        self.ui.setupUi(self)
        self.ui.buttonBox.accepted.connect(lambda: self.handle_login(servers=servers))
        servers = {}
        with open('servers.csv', newline='') as csvfile:
            server_reader = csv.reader(csvfile)
            for row in server_reader:
                self.ui.cbo_db_name.addItem(row[1])
                servers[row[1]] = (row[0],row[2])

    def handle_login(self, servers=''):
        global user
        global pword
        global database
        global server
        global bg_colour
        user = self.ui.username.text()
        pword = self.ui.password.text()
        database = self.ui.cbo_db_name.currentText()
        server = servers[database][0]
        bg_colour = servers[database][1]


if __name__=="__main__":
    app=QApplication(sys.argv)
    global hotdate
    global hotdate_string
    global folio_num
    global user
    global pword
    global dbase
    global server
    pword = ""
    global database
    global bg_colour
    #Login
    while True:
        if Login().exec_() == QDialog.Accepted:
            db = QSqlDatabase.addDatabase("QPSQL");
            db.setHostName(server)
            db.setDatabaseName(database);
            db.setUserName(user);
            db.setPassword(pword)
            if (db.open()==False):     
                QMessageBox.critical(None, "Database Error", db.lastError().text())
            else:
                break
        else:
            #QApplication.quit()
            QCoreApplication.instance().quit()            
            #sys.exit()


    myapp = Ci_Co()
    myapp.show()
    sys.exit(app.exec_())

Answer 1

Calling QCoreApplication.quit() is the same as calling QCoreApplication.exit(0). To quote from the qt docs:

After this function has been called, the application leaves the main event loop and returns from the call to exec(). The exec() function returns returnCode. If the event loop is not running, this function does nothing. [emphasis added]

So quit() or exit() are nothing like sys.exit(). The latter will terminate the program, but the former will merely terminate the event-loop (if it's running).

When the user cancels the login dialog, your example should just call sys.exit() to terminate the program. Otherwise, your program will just get stuck in the blocking while-loop.