code: def to_str(value):
      
      if sys.version_info.major < 3 and isinstance(value, six.string_types):
          return value
      return str(value)
docs: Convert the input to a string, unless it is a unicode string in Python 2.
  
      Unicode strings are supported as native strings in Python 3, but ``str()`` cannot be
      invoked on unicode strings in Python 2, so we need to check for that case when
      converting user-specified values to strings.
  
      Args:
          value: The value to convert to a string.
  
      Returns:
          str or unicode: The string representation of the value or the unicode string itself.