Chameleon template to alternate class="odd" or "even"

gistfile1.xml

        <tbody>
          <tal:block repeat="ur url_recs">
            <tr tal:define="odd repeat.ur.odd"
                tal:attributes="class python: odd and 'odd' or 'even'">
              <td>${ur.datetime}</td>
              <td>...</td>
            </tr>
          </tal:block>
        </tbody>