docs: def clear(self, url=None, xpath=None):
          """Clear cache
  
          Args:
              url (str): If given, clear specific item only. Otherwise remove the DB file.
              xpath (str): xpath to search (may be ``None``)
          """
          if url is not None:
              query = self._query(url, xpath)
              if query.count() > 0:
                  query.delete()
                  self.session.commit()
              else:
                  raise KeyError("Cannot clear URL, not in cache: " + str(url) + " xpath:" + str(xpath))
          else:
              # remove the DB file
              self.close()
              if path.exists(self.db_path):
                  remove(self.db_path)