code: def _get_sub_prop(self, key, default=None):
          
          return _helpers._get_sub_prop(
              self._properties, [self._job_type, key], default=default
          )
docs: Get 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 access
          those properties::
  
              self._get_sub_prop('destinationTable')
  
          This is equivalent to using the ``_helpers._get_sub_prop`` function::
  
              _helpers._get_sub_prop(
                  self._properties, ['query', 'destinationTable'])
  
          Arguments:
              key (str):
                   Key for the value to get in the
                   ``self._properties[self._job_type]`` dictionary.
              default (object):
                  (Optional) Default value to return if the key is not found.
                  Defaults to :data:`None`.
  
          Returns:
              object: The value if present or the default.