<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def python_portable_string(string, encoding='utf-8'):
 """Converts bytes into a string type. Valid string types are retuned without modification. So in Python 2, type str and unicode are not converted. In Python 3, type bytes is converted to type str (unicode) """

  if isinstance(string, six.string_types):
    return string

  if six.PY3:
    return string.decode(encoding)

  raise ValueError('Unsupported type %s' % str(type(string)))