PyQt5 / PySide2 QT StyleSheets (qss) dynamic "class" property example

main.py

from PySide2.QtCore import QObject, QTimer
from PySide2.QtWidgets import QApplication, QDialog
import sys


class Form(QDialog):
    def __init__(self, parent=None):
        super(Form, self).__init__(parent)
        self.setWindowTitle("My Form")
        self.setStyleSheet("""
            .active {
                background-color: red;
            }
            """)

        self.timer = QTimer()
        self.timer.timeout.connect(self.onTimer)
        self.timer.start(1000)

    def onTimer(self):
        self.toggleCssClass('active')

    def hasCssClass(self, className):
        classNames = self.readCssClasses()
        return className in classNames

    def removeCssClass(self, className):
        classNames = self.readCssClasses()
        if className in classNames:
            classNames.remove(className)
        self.setCssClasses(classNames)

    def addCssClass(self, className):
        classNames = self.readCssClasses()
        classNames.add(className)
        self.setCssClasses(classNames)

    def toggleCssClass(self, className):
        if self.hasCssClass(className):
            self.removeCssClass(className)
        else:
            self.addCssClass(className)

    def readCssClasses(self) -> set:
        classStr = self.property('class')
        if classStr is None:
            classStr = ''
        classStr = classStr.strip()
        classes = []
        if len(classStr) > 0:
            classes = classStr.split(' ')
        return set(classes)

    def setCssClasses(self, classNames: set):
        current = self.readCssClasses()
        if (current != classNames):
            self.setProperty('class', ''.join(classNames))
            self.style().unpolish(self)
            self.style().polish(self)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    form = Form()
    form.show()
    sys.exit(app.exec_())