Intro Graphs | Python Fiddle

""" A Python Class
A simple Python graph class, demonstrating the essential 
facts and functionalities of graphs.
"""

class Graph(object):

def __init__(self, graph_dict=None):
        """ initializes a graph object 
            If no dictionary or None is given, 
            an empty dictionary will be used
        """
        if graph_dict == None:
            graph_dict = {}
        self.__graph_dict = graph_dict

def vertices(self):
        """ returns the vertices of a graph """
        return list(self.__graph_dict.keys())

def edges(self):
        """ returns the edges of a graph """
        return self.__generate_edges()

def add_vertex(self, vertex):
        """ If the vertex "vertex" is not in 
            self.__graph_dict, a key "vertex" with an empty
            list as a value is added to the dictionary. 
            Otherwise nothing has to be done. 
        """
        if vertex not in self.__graph_dict:
            self.__graph_dict[vertex] = []

def add_edge(self, edge):
        """ assumes that edge is of type set, tuple or list; 
            between two vertices can be multiple edges! 
        """
        edge = set(edge)
        (vertex1, vertex2) = tuple(edge)
        if vertex1 in self.__graph_dict:
            self.__graph_dict[vertex1].append(vertex2)
        else:
            self.__graph_dict[vertex1] = [vertex2]

def __generate_edges(self):
        """ A static method generating the edges of the 
            graph "graph". Edges are represented as sets 
            with one (a loop back to the vertex) or two 
            vertices 
        """
        edges = []
        for vertex in self.__graph_dict:
            for neighbour in self.__graph_dict[vertex]:
                if {neighbour, vertex} not in edges:
                    edges.append({vertex, neighbour})
        return edges

def __str__(self):
        res = "vertices: "
        for k in self.__graph_dict:
            res += str(k) + " "
        res += "\nedges: "
        for edge in self.__generate_edges():
            res += str(edge) + " "
        return res

if __name__ == "__main__":

g = { "a" : ["d"],
          "b" : ["c"],
          "c" : ["b", "c", "d", "e"],
          "d" : ["a", "c"],
          "e" : ["c"],
          "f" : []
        }

graph = Graph(g)

print("Vertices of graph:")
    print(graph.vertices())

print("Edges of graph:")
    print(graph.edges())

print("Add vertex:")
    graph.add_vertex("z")

print("Vertices of graph:")
    print(graph.vertices())
 
    print("Add an edge:")
    graph.add_edge({"a","z"})
    
    print("Vertices of graph:")
    print(graph.vertices())

print("Edges of graph:")
    print(graph.edges())

print('Adding an edge {"x","y"} with new vertices:')
    graph.add_edge({"x","y"})
    print("Vertices of graph:")
    print(graph.vertices())
    print("Edges of graph:")
    print(graph.edges())

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