simple_graph.py

""" A Python Class
A simple Python graph class. https://repl.it/GyUb/0
"""

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())


def dfs(graph, start):
    visited, stack = set(), [start]
    while stack:
        vertex = stack.pop()
        if vertex not in visited:
            visited.add(vertex)
            stack.extend(set(graph.graphdict[vertex]) - visited)
    return visited

If you start this module standalone, you will get the following result:
$ python3 graph.py
Vertices of graph:
['a', 'c', 'b', 'e', 'd', 'f']
Edges of graph:
[{'a', 'd'}, {'c', 'b'}, {'c'}, {'c', 'd'}, {'c', 'e'}]
Add vertex:
Vertices of graph:
['a', 'c', 'b', 'e', 'd', 'f', 'z']
Add an edge:
Vertices of graph:
['a', 'c', 'b', 'e', 'd', 'f', 'z']
Edges of graph:
[{'a', 'd'}, {'c', 'b'}, {'c'}, {'c', 'd'}, {'c', 'e'}, {'a', 'z'}]
Adding an edge {"x","y"} with new vertices:
Vertices of graph:
['a', 'c', 'b', 'e', 'd', 'f', 'y', 'z']
Edges of graph:
[{'a', 'd'}, {'c', 'b'}, {'c'}, {'c', 'd'}, {'c', 'e'}, {'a', 'z'}, {'y', 'x'}]