code: def _set_sub_prop(self, key, value):
          
          _helpers._set_sub_prop(self._properties, [self._job_type, key], value)
docs: Set a value in the ``self._properties[self._job_type]`` dictionary.
  
          Most job properties are inside the dictionary related to the job type
          (e.g. 'copy', 'extract', 'load', 'query'). Use this method to set
          those properties::
  
              self._set_sub_prop('useLegacySql', False)
  
          This is equivalent to using the ``_helper._set_sub_prop`` function::
  
              _helper._set_sub_prop(
                  self._properties, ['query', 'useLegacySql'], False)
  
          Arguments:
              key (str):
                   Key to set in the ``self._properties[self._job_type]``
                   dictionary.
              value (object): Value to set.