<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def cachedir_index_del(minion_id, base=None):
    '''
    Delete an entry from the cachedir index. This generally only needs to happen
    when an instance is deleted.
    '''
    base = init_cachedir(base)
    index_file = os.path.join(base, 'index.p')
    lock_file(index_file)

    if os.path.exists(index_file):
        mode = 'rb' if six.PY3 else 'r'
        with salt.utils.files.fopen(index_file, mode) as fh_:
            index = salt.utils.data.decode(
                salt.utils.msgpack.load(fh_, encoding=MSGPACK_ENCODING))
    else:
        return

    if minion_id in index:
        del index[minion_id]

    mode = 'wb' if six.PY3 else 'w'
    with salt.utils.files.fopen(index_file, mode) as fh_:
        salt.utils.msgpack.dump(index, fh_, encoding=MSGPACK_ENCODING)

    unlock_file(index_file)