This is the code snippet below: from math import inf import networlot as nx class DiGraphWithEdgeCheckerinxDiGraph): ln NetworkX's implementation of add_edge. the nodes u and y will be automatically added if they are not already in the graph. This subclass will fail-fast if you try to do that. which may help you catch mistakes more easily. We also want you to practice defining your yertex set explicitly instead of implicitly creating yertices when you add edges. def add_edge[self. u_of_edge, y_of_edge. ' *attr]: if not [u_of_edge in self. nodes and y_of_edge in self.nodes]: raise Exceptioni"Endpoints of edge are not in the graph's yertex set") super[}.add_edge[u_of_edge. y_of_edge. " *attr} answer =