code: def diff_op(self):
          
          if self.graph is not None:
              if isinstance(self.graph, graphtools.graphs.LandmarkGraph):
                  diff_op = self.graph.landmark_op
              else:
                  diff_op = self.graph.diff_op
              if sparse.issparse(diff_op):
                  diff_op = diff_op.toarray()
              return diff_op
          else:
              raise NotFittedError("This PHATE instance is not fitted yet. Call "
                                   "'fit' with appropriate arguments before "
                                   "using this method.")
docs: The diffusion operator calculated from the data