Python - Metodos grafos

class Graph:

    def __init__(self,n = None, e = None):

        self._nodes = {} if not n else n

        self._edges = {} if not n else e

    @property

    def node(self):

        return self._nodes

    @property

    def edge(self):

        return self._edges

    def nodes(self):

        return [node for node in self._nodes.keys()]

    def edges(self):

        edges = []

        for node1, edge_list in self._edges.items():

            for node2 in edge_list.keys():

                possible_edge = (node1,node2) if node1 <= node2 else (node2,node1)

                #repeticiones

                if possible_edge not in edges:

                    edges.append(possible_edge)

        return edges

    def add_node(self, node, attr_dict=None):

        self._nodes[node] = attr_dict if attr_dict else {} # si esta vacio, anade un diccionario

    def add_edge(self, node1, node2, attr_dict=None):

        if node1 not in self._nodes.keys(): # si no hay nodos los creamos

            self.add_node(node1)

        if node2 not in self._nodes.keys():

            self.add_node(node2)

        if attr_dict:

            self._edges.setdefault(node1,{})

        else:

            self._edges.setdefault(node1,{})[node2] = {}

            self._edges.setdefault(node2,{})[node1] = {}

    # def add_nodes_from(self, node_list, attr_dict=None):

        # pass

    # def add_edges_from(self, edge_list, attr_dict=None):

        # pass