code: def is_unicode_string(string):
      
      if string is None:
          return None
      if PY2:
          return isinstance(string, unicode)
      return isinstance(string, str)
docs: Return ``True`` if the given string is a Unicode string,
      that is, of type ``unicode`` in Python 2 or ``str`` in Python 3.
  
      Return ``None`` if ``string`` is ``None``.
  
      :param str string: the string to be checked
      :rtype: bool