How to keep open Tkcalendar after running code

The problem is that you are using 2 libraries that have 2 eventloops, a possible solution is to run the eventloops separately in different processes but that is unnecessarily complicating the application. A better solution is to write all the logic using a single library, in this one because there is no equivalent of Qt WebEngine in tkinter I will choose to use pyqt5:

from PyQt5 import QtCore, QtWidgets, QtWebEngineWidgetsSCRIPT_FIND_URL = """var url = ""var phrase = "Tipo de cambio para solventar obligaciones denominadas en moneda extranjera pagaderas en la República Mexicana."var elements = document.getElementsByTagName("a");for(const e of elements){    if(e.text.includes(phrase))        url = e.href}url"""class PageOffline(QtWebEngineWidgets.QWebEnginePage):    finished = QtCore.pyqtSignal(bool)    def __init__(self, parent=None):        super().__init__(parent)        self.date_url = QtCore.QUrl()        self.loadFinished.connect(self.handle_loaded)        self.pdfPrintingFinished.connect(self.handle_pdf)    def search(self, date):        self.date = date        self.date_url = QtCore.QUrl("https://www.dof.gob.mx/index_111.php")        query = QtCore.QUrlQuery()        query.addQueryItem("year", self.date.toString("yyyy"))        query.addQueryItem("month", self.date.toString("MM"))        query.addQueryItem("day", self.date.toString("dd"))        query.addQueryItem("edicion", "MAT")        self.date_url.setQuery(query)        self.load(self.date_url)    def handle_loaded(self, ok):        if ok:            if self.url() == self.date_url:                self.runJavaScript(SCRIPT_FIND_URL, self.handle_url)            else:                filename = "{}.pdf".format(self.date.toString("yyyy-MM-dd"))                self.printToPdf(filename)        else:            self.finished.emit(False)    def handle_url(self, url):        if url:            pdf_url = QtCore.QUrl.fromUserInput(url)            self.load(pdf_url)        else:            self.finished.emit(False)    def handle_pdf(self, path, ok):        self.finished.emit(ok)class Widget(QtWidgets.QWidget):    def __init__(self, parent=None):        super().__init__(parent)        self.page = PageOffline()        self.button = QtWidgets.QPushButton("Generate pdf")        self.calendar = QtWidgets.QCalendarWidget()        lay = QtWidgets.QVBoxLayout(self)        lay.addWidget(self.calendar)        lay.addWidget(self.button, alignment=QtCore.Qt.AlignCenter)        self.button.clicked.connect(self.handle_clicked)        self.page.finished.connect(self.handle_print_finished)    def handle_clicked(self):        date = self.calendar.selectedDate()        self.page.search(date)        self.button.setEnabled(False)    def handle_print_finished(self, status):        QtWidgets.QMessageBox.information(            self,            "Generación de PDF",            "El PDF fue generado con éxito" if status else "La generación de PDF fallo",        )        self.button.setEnabled(True)def main():    import sys    app = QtWidgets.QApplication(sys.argv)    QtCore.QLocale.setDefault(QtCore.QLocale(QtCore.QLocale.Spanish))    w = Widget()    w.resize(400, 400)    w.show()    sys.exit(app.exec_())if __name__ == "__main__":    main()