Python Program to Check the Connectivity of Undirected Graph Using DFS

class Graph:
    def __init__(self):
        # dictionary containing keys that map to the corresponding vertex object
        self.vertices = {}
 
    def add_vertex(self, key):
        """Add a vertex with the given key to the graph."""
        vertex = Vertex(key)
        self.vertices[key] = vertex
 
    def get_vertex(self, key):
        """Return vertex object with the corresponding key."""
        return self.vertices[key]
 
    def __contains__(self, key):
        return key in self.vertices
 
    def add_edge(self, src_key, dest_key, weight=1):
        """Add edge from src_key to dest_key with given weight."""
        self.vertices[src_key].add_neighbour(self.vertices[dest_key], weight)
 
    def does_edge_exist(self, src_key, dest_key):
        """Return True if there is an edge from src_key to dest_key."""
        return self.vertices[src_key].does_it_point_to(self.vertices[dest_key])
 
    def add_undirected_edge(self, v1_key, v2_key, weight=1):
        """Add undirected edge (2 directed edges) between v1_key and v2_key with
        given weight."""
        self.add_edge(v1_key, v2_key, weight)
        self.add_edge(v2_key, v1_key, weight)
 
    def does_undirected_edge_exist(self, v1_key, v2_key):
        """Return True if there is an undirected edge between v1_key and v2_key."""
        return (self.does_edge_exist(v1_key, v2_key)
                and self.does_edge_exist(v1_key, v2_key))
 
    def __iter__(self):
        return iter(self.vertices.values())
 
 
class Vertex:
    def __init__(self, key):
        self.key = key
        self.points_to = {}
 
    def get_key(self):
        """Return key corresponding to this vertex object."""
        return self.key
 
    def add_neighbour(self, dest, weight):
        """Make this vertex point to dest with given edge weight."""
        self.points_to[dest] = weight
 
    def get_neighbours(self):
        """Return all vertices pointed to by this vertex."""
        return self.points_to.keys()
 
    def get_weight(self, dest):
        """Get weight of edge from this vertex to dest."""
        return self.points_to[dest]
 
    def does_it_point_to(self, dest):
        """Return True if this vertex points to dest."""
        return dest in self.points_to
 
 
def label_all_reachable(vertex, component, label):
    """Set component[v] = label for all v in the component containing vertex."""
    label_all_reachable_helper(vertex, set(), component, label)
 
 
def label_all_reachable_helper(vertex, visited, component, label):
    """Set component[v] = label for all v in the component containing
    vertex. Uses set visited to keep track of nodes alread visited."""
    visited.add(vertex)
    component[vertex] = label
    for dest in vertex.get_neighbours():
        if dest not in visited:
            label_all_reachable_helper(dest, visited, component, label)
 
 
g = Graph()
print('Undirected Graph')
print('Menu')
print('add vertex <key>')
print('add edge <src> <dest>')
print('components')
print('display')
print('quit')
 
while True:
    do = input('What would you like to do? ').split()
 
    operation = do[0]
    if operation == 'add':
        suboperation = do[1]
        if suboperation == 'vertex':
            key = int(do[2])
            if key not in g:
                g.add_vertex(key)
            else:
                print('Vertex already exists.')
        elif suboperation == 'edge':
            src = int(do[2])
            dest = int(do[3])
            if src not in g:
                print('Vertex {} does not exist.'.format(src))
            elif dest not in g:
                print('Vertex {} does not exist.'.format(dest))
            else:
                if not g.does_undirected_edge_exist(src, dest):
                    g.add_undirected_edge(src, dest)
                else:
                    print('Edge already exists.')
 
    elif operation == 'components':
        component = dict.fromkeys(g, None)
        label = 1
        for v in g:
            if component[v] is None:
                label_all_reachable(v, component, label)
                label += 1
 
        max_label = label
        for label in range(1, max_label):
            print('Component {}:'.format(label),
                  [v.get_key() for v in component if component[v] == label])
 
 
    elif operation == 'display':
        print('Vertices: ', end='')
        for v in g:
            print(v.get_key(), end=' ')
        print()
 
        print('Edges: ')
        for v in g:
            for dest in v.get_neighbours():
                w = v.get_weight(dest)
                print('(src={}, dest={}, weight={}) '.format(v.get_key(),
                                                             dest.get_key(), w))
        print()
 
    elif operation == 'quit':
        break