code: def _add_edge(self, source, target, **kwargs):
          
          # Start with default edge properties
          edge_properties = self.edge_properties
          # Overwrite ones that are given in function call explicitly
          for k, v in kwargs.items():
              edge_properties[k] = v
          self.graph.add_edge(source, target, **edge_properties)
docs: Add an edge to the graph.