Сигналы и слоты в PyQt5

Пример тестового интерфейса с описанием подключения слота и сигнала, но оно не срабатывает по непонятным причинам.

import sys, binascii, glob, asyncio 
from PyQt5.QtCore import QObject, pyqtSlot, QDateTime, Qt, QTimer 
from PyQt5 import QtCore, QtGui, QtWidgets 
from PyQt5.QtWidgets import (QApplication, QCheckBox, QComboBox, QDateTimeEdit,
        QDial, QDialog, QGridLayout, QGroupBox, QHBoxLayout, QLabel, QLineEdit,
        QProgressBar, QPushButton, QRadioButton, QScrollBar, QSizePolicy,
        QSlider, QSpinBox, QStyleFactory, QTableWidget, QTabWidget, QTextEdit,
        QVBoxLayout, QWidget, QMainWindow, QMessageBox) 

class Ui_MainWindow(object): 
    def setupUi(self, MainWindow): 
        MainWindow.setObjectName("MainWindow"); MainWindow.resize(400, 300);
        self.centralWidget = QtWidgets.QWidget(MainWindow); self.centralWidget.setObjectName("centralWidget");
        self.gridLayout = QtWidgets.QGridLayout(self.centralWidget); self.gridLayout.setContentsMargins(11,11,11,11);
        self.gridLayout.setSpacing(6); self.gridLayout.setObjectName("gridLayout");
        self.verticalLayout = QtWidgets.QVBoxLayout(); self.verticalLayout.setSpacing(6);
        self.verticalLayout.setObjectName("verticalLayout");
        self.pushButtonFirst = QtWidgets.QPushButton(self.centralWidget);
        self.pushButtonFirst.setObjectName("pushButtonFirst");self.verticalLayout.addWidget(self.pushButtonFirst);
        self.pushButtonSecond = QtWidgets.QPushButton(self.centralWidget);
        self.pushButtonSecond.setObjectName("pushButtonSecond");self.verticalLayout.addWidget(self.pushButtonSecond);
        self.checkBox = QtWidgets.QCheckBox(self.centralWidget);
        self.checkBox.setObjectName("checkBox"); self.verticalLayout.addWidget(self.checkBox);
        self.lineEdit = QtWidgets.QLineEdit(self.centralWidget);
        self.lineEdit.setObjectName("lineEdit"); self.verticalLayout.addWidget(self.lineEdit);
        self.gridLayout.addLayout(self.verticalLayout,0,0,1,1);MainWindow.setCentralWidget(self.centralWidget); 
        self.menuBar = QtWidgets.QMenuBar(MainWindow);self.menuBar.setGeometry(QtCore.QRect(0,0,400,21));
        self.menuBar.setObjectName("menuBar");MainWindow.setMenuBar(self.menuBar);
        self.mainToolBar = QtWidgets.QToolBar(MainWindow);self.mainToolBar.setObjectName("mainToolBar");
        MainWindow.addToolBar(QtCore.Qt.TopToolBarArea, self.mainToolBar);
        self.statusBar = QtWidgets.QStatusBar(MainWindow);self.statusBar.setObjectName("statusBar");
        MainWindow.setStatusBar(self.statusBar);self.retranslateUi(MainWindow);
        QtCore.QMetaObject.connectSlotsByName(MainWindow);

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButtonFirst.setText(_translate("MainWindow", "PushButton"))
        self.pushButtonSecond.setText(_translate("MainWindow", "PushButton"))
        self.checkBox.setText(_translate("MainWindow", "CheckBox"))

class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent); self.setupUi(self);
        self.pushButtonFirst.released.connect(self.pushButtonClicked)
        #self.pushButtonFirst.clicked.connect(self.pushButtonClicked) 

    @QtCore.pyqtSlot() 
    def pushButtonClicked(self):
        try:
            sender = self.sender();
            self.statusBar().showMessage(sender.text() + ' was pressed')
            msg = QMessageBox();
            msg.setIcon(QMessageBox.Information);
            msg.setText("This is a message box");
            msg.setInformativeText("This is additional information");
            msg.setWindowTitle("MessageBox demo");
            msg.setDetailedText("The details are as follows:");
            msg.setStandardButtons(QMessageBox.Ok | QMessageBox.Cancel);
            msg.exec_(); 
        except  Exception as e: 
            return str(e);

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