def unique_courses(slots_table):
    """Obtain list of unique courses from slot listing.

    Arguments:
        slots_table (list of dict) : TA slot records
    
    Returns:
        (list of str) : sorted list of course numbers

    """

    # obtain list of unique course numbers
    course_set = set()
    for slot in slots_table:
        course_set.add(slot["course"])
    course_list = list(course_set)
    course_list.sort()

    return course_list