code: def reduce(self, func, dim=None, keep_attrs=None, **kwargs):
          
          if dim == DEFAULT_DIMS:
              dim = ALL_DIMS
              # TODO change this to dim = self._group_dim after
              # the deprecation process. Do not forget to remove _reduce_method
              warnings.warn(
                  "Default reduction dimension will be changed to the "
                  "grouped dimension in a future version of xarray. To "
                  "silence this warning, pass dim=xarray.ALL_DIMS "
                  "explicitly.",
                  FutureWarning, stacklevel=2)
          elif dim is None:
              dim = self._group_dim
  
          if keep_attrs is None:
              keep_attrs = _get_keep_attrs(default=False)
  
          def reduce_dataset(ds):
              return ds.reduce(func, dim, keep_attrs, **kwargs)
          return self.apply(reduce_dataset)
docs: Reduce the items in this group by applying `func` along some
          dimension(s).
  
          Parameters
          ----------
          func : function
              Function which can be called in the form
              `func(x, axis=axis, **kwargs)` to return the result of collapsing
              an np.ndarray over an integer valued axis.
          dim : str or sequence of str, optional
              Dimension(s) over which to apply `func`.
          axis : int or sequence of int, optional
              Axis(es) over which to apply `func`. Only one of the 'dimension'
              and 'axis' arguments can be supplied. If neither are supplied, then
              `func` is calculated over all dimension for each group item.
          keep_attrs : bool, optional
              If True, the datasets's attributes (`attrs`) will be copied from
              the original object to the new one.  If False (default), the new
              object will be returned without attributes.
          **kwargs : dict
              Additional keyword arguments passed on to `func`.
  
          Returns
          -------
          reduced : Array
              Array with summarized data and the indicated dimension(s)
              removed.