def verification(self):
    username = username_verify.get()
    password = password_verify.get()

    username_entry.delete(0, END)
    password_entry.delete(0, END)

    import csv
    with open('Users.csv') as csvfile:
        reader = csv.DictReader(csvfile)
        database = []
        for row in reader:
            database.append(dict(username=row['username'], password=row['password']))

    logged_in = False
    while not logged_in:
        for row in database:
            username_file = row['username']
            password_file = row['password']

            if (username_file == username and password_file == password):
                logged_in = True
                tkinter.messagebox.showwarning("Success", "Login successful")
                root.destroy()
                import SecondWindow
            elif username == "" or password == "":
                tkinter.messagebox.showwarning("Error", "Username or password not entered, please retry!")
            else:
                tkinter.messagebox.showwarning("Error", "Username or password not recognised, please retry!")