How to properly test a function that fill a combobox from a database

I am beginner in Python, and I am in front of a smal problem. I am using PyQt5 for my IHM. It is a form. I need to fill some combobox with datas in a database.

For the moment, it's in a class name Ui_dialog. I have two questions.

1. How can I properly make an unit test on these function? I do not really know how to start my tests sometimes...
2. Does I need to use the ressources of my project ( database, xmls... ) or I need to make some specials datas for my test in order cover all possible cases ?

        def fill_combobox_from_database(self,name_of_table,name_of_field,combobox):
            connexion = sqlite3.connect(os.path.dirname(os.path.abspath(__file__))+"\\"+Ui_dialog.NAME_DB)
            cursor = connexion.cursor()
            try:
                request = "select {0} from {1}".format(name_of_field,name_of_table)
                results = cursor.execute(request)
                for row in results:
                    combobox.addItem(row[0])
            except Exception as e:
                print(e)

Thanks for reading me and for helping me!