code: def pre_filter(self):
          
          if self._name in self.PRE_FILTER_FIELDS:
              if not self._value:
                  return '"not=${}"'.format(self.PRE_FILTER_FIELDS[self._name])
              else:
                  val = self._value
                  if self._exact:
                      val = val.copy().pop()
                  return r'"string.contains_i=${},\"{}\""'.format(
                         self.PRE_FILTER_FIELDS[self._name], val.replace('"', r'\\\"'))
          return ''
docs: Return rTorrent condition to speed up data transfer.