<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def utf8(value):
 r""" Check that the string is UTF-8. Returns an encode bytestring. Traceback (most recent call last):
 """

    try:
        if isinstance(value, bytes):
            return value.decode('utf-8')
        else:
            return value
    except Exception:
        raise ValueError('Not UTF-8: %r' % value)