code: def sum(self, axis=0, dtype=None):
          
          if axis not in [0, 1, 'index', 'columns']:
              raise ValueError('"axis" parameter must be one of 0, 1, "index", '
                               'or "columns".')
  
          sum_kwargs = {}
          if dtype is not None:
              sum_kwargs['dtype'] = dtype
          y = self.values.sum(axis=axis, **sum_kwargs)
  
          if axis == 0 or axis == 'index':
              y = profile.ExpProfile(y, genes=self.cells)
          else:
              y = profile.ExpProfile(y, genes=self.genes)
  
          return y
docs: Sum over rows or columns.
          
          Overrides the default `pandas.DataFrame.sum()` function to prevent
          temporary in-memory copies.