docs: def groupby(self, by):
          """Group by certain columns, excluding index.
  
          Simply reset_index if desiring to group by some index column too.
  
          Parameters
          ----------
          by  : str or list of str
              Column(s) to groupby.
  
          Returns
          -------
          DataFrameGroupBy
              Object encoding the groupby operation.
  
          """
          check_str_or_list_str(by)
          by = as_list(by)
          if len(set(by)) == len(self._data):
              raise ValueError('Cannot groupby all columns')
  
          from .groupby import DataFrameGroupBy
  
          return DataFrameGroupBy(self, by)