def list_add(instructor, instructor_list=[]):
    """ Add another instructor, use [] as default value for instructor_list.
    """
    instructor_list.append(instructor)
    print(instructor_list)
    return instructor_list


def list_add2(instructor, instructor_list=None):
    """ Add another instructor, use [] as default value for instructor_list.
    """
    if instructor_list is None:
        instructor_list = []
    instructor_list.append(instructor)
    print(instructor_list)
    return instructor_list


if __name__ == "__main__":
    list_add("bogdan")
    list_add("danny")
    list_add("jacqueline")

    list_add2("bogdan")
    list_add2("danny")
    list_add2("jacqueline")