'Check whether the field type is a string type (lazy property)
@return: True if field type is string or text, else False'

@property
def is_string(self):

    is_string = self._is_string
    if (is_string is None):
        is_string = (self.ftype in ('string', 'text'))
        self._is_string = is_string
    return is_string
