How to go about monitoring change?

Question

Backstory:

I'm currently developing a program that has several qComboBox and qLineEdit elements that a user can enter data into or select a value. When the user selects "New File" or "Open File" I want to check if any values have changed, and present the user with the option to save their work. The output file is XML.

Problem:

Basically, I just need to know if any of the values are different than the default values. I'm not concerned with what exact values are different, I just need to know that they are different.

I have tried using xmldiff by creating an Element Tree that contains the initial values when the program starts, then comparing that to a second Element Tree with the current values. It does not appear to be capable of just giving a true or false value, and the second Element Tree varies in size so I don't think I can simply do a 1 for 1 comparison.

The second thing I tried was simply setting a boolean value when the element changed, but I couldn't account for an element being reset to a default value. For example if a qLineEdit box had no value, and a user input something, that would set the boolean value to "true"; however, if they went back in and changed that value back to the default value the result would be "true" as well.

I was wondering if there is a "best practice" for doing this type of thing, or if someone could point me in the right direction. This seems like it should be trivial for the most part, but I don't know how to approach this.

EDIT … Added example for second attempt.

import sys
from PyQt5.QtWidgets import *


class Window(QWidget):
    def __init__(self):
        super(Window, self).__init__()

        self.modified = False
        self.edit1 = QLineEdit()
        layout = QGridLayout(self)
        layout.addWidget(self.edit1)

        self.edit1.editingFinished.connect(self.valueChanged)

    def valueChanged(self):
        print('valueChanged Event')
        self.modified = True

    def closeEvent(self, event):
        if self.modified:
            prompt = QMessageBox.warning(
                self, 'Save Changes?',
                'This document has been modified.\n'
                'Do you want to save these changes?',
                QMessageBox.Yes | QMessageBox.No | QMessageBox.Cancel, QMessageBox.Cancel
        )

            if prompt == QMessageBox.Yes:
                event.accept()
            if prompt == QMessageBox.No:
                event.accept()
            if prompt == QMessageBox.Cancel:
                event.ignore()


if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = Window()
    window.setGeometry(600, 100, 300, 100)
    window.show()
    sys.exit(app.exec_())

Answer 1

Member-dict with defaults:

Create a dict with { qline1Name: "tata", qcombo: 2, ...}. It holds your instances as key and its inital value. On save check if any current value is different from what stored in it and act accordingly. You would have to differentiate a couple of classes used as input elements - for combobox the initial selected index could be used.

Derive / monkey patch / decorate:

You could derive / monkey patch / decorate your Q*-inputs and add a default_value property to them. Set it on creation and evaluate on save:

• How to add property to a class dynamically?
• Monkey-patch Python class
• How to make a chain of function decorators?

(Mis-)use existing fields:

QLineEdit has QLineEdit.placeholderText that you could use for the "defaultvalue" and simply compare it (unless your defaultText are spaces - they get nixx'ed).

A more generic approach would be to leverage the QWidget.whatsThis QString field. That way only one property can be used for all input elements and you can if isinstance(...): to cast it's QString value when comparing it to the actual one.