<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def unicode_string(string):
 """ Make sure string is unicode, try to default with utf8, or base64 if failed. can been decode by `decode_unicode_string` """

    if isinstance(string, six.text_type):
        return string
    try:
        return string.decode("utf8")
    except UnicodeDecodeError:
        return '[BASE64-DATA]' + base64.b64encode(string) + '[/BASE64-DATA]'