from PyQt5 import QtCore, QtGui, QtWidgets  # Import the PyQt5 module we'll need
import sys  # We need sys so that we can pass argv to QApplication

import design  # This file holds our MainWindow and all design related things

# it also keeps events etc that we defined in Qt Designer
import os  # For listing directory methods

class ExampleApp(QtWidgets.QMainWindow, design.Ui_MainWindow):
    def __init__(self):
        # Explaining super is out of the scope of this article
        # So please google it if you're not familar with it
        # Simple reason why we use it here is that it allows us to
        # access variables, methods etc in the design.py file
        super(self.__class__, self).__init__()
        self.setupUi(self)  # This is defined in design.py file automatically
        # It sets up layout and widgets that are defined
        self.btnBrowse.clicked.connect(self.browse_folder)  # When the button is pressed
                                                            # Execute browse_folder function

    def browse_folder(self):
        self.listWidget.clear() # In case there are any existing elements in the list
        directory = QtGui.QFileDialog.getExistingDirectory(self,
                                                           "Pick a folder")
        # execute getExistingDirectory dialog and set the directory variable to be equal
        # to the user selected directory

        if directory: # if user didn't pick a directory don't continue
            for file_name in os.listdir(directory): # for all files, if any, in the directory
                self.listWidget.addItem(file_name)  # add file to the listWidget

def main():
    app = QtGui.QApplication(sys.argv)  # A new instance of QApplication
    form = ExampleApp()  # We set the form to be our ExampleApp (design)
    form.show()  # Show the form
    app.exec_()  # and execute the app

if __name__ == '__main__':  # if we're running file directly and not importing it
    main()  # run the main function

I get the error :
app = QtGui.QApplication(sys.argv) AttributeError : 'module' object has no attribute 'QApplication'

I have a "design.py" which is the gui of my application. This code is my "main.py" that's suppose to show and execute the app. But I got that error. Not sure what is wrong. Thank you in advance.