'Extract a field value from a DAL row
@param row: the row
@param field: the fieldname (list_fields syntax)'

def _extract(self, row, field):

    rfields = self.rfields
    if (field not in rfields):
        raise KeyError(('Invalid    field    name:    %s' % field))
    rfield = rfields[field]
    try:
        return rfield.extract(row)
    except AttributeError:
        return None
