Detect Cycle in Directed Graph using DFS in Python

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 __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 is_cycle_present(graph):
    """Return True if cycle is present in the graph."""
    on_stack = set()
    visited = set()
    for v in graph:
        if v not in visited:
            if is_cycle_present_helper(v, visited, on_stack):
                return True
    return False
 
 
def is_cycle_present_helper(v, visited, on_stack):
    """Return True if the DFS traversal starting at vertex v detects a
    cycle. Uses set visited to keep track of nodes that have been visited. Uses
    set on_stack to keep track of nodes that are 'on the stack' of the recursive
    calls."""
    if v in on_stack:
        return True
    on_stack.add(v)
    for dest in v.get_neighbours():
        if dest not in visited:
            if is_cycle_present_helper(dest, visited, on_stack):
                return True
    on_stack.remove(v)
    visited.add(v)
    return False
 
 
g = Graph()
print('Menu')
print('add vertex <key>')
print('add edge <vertex1> <vertex2>')
print('cycle')
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':
            v1 = int(do[2])
            v2 = int(do[3])
            if v1 not in g:
                print('Vertex {} does not exist.'.format(v1))
            elif v2 not in g:
                print('Vertex {} does not exist.'.format(v2))
            else:
                if not g.does_edge_exist(v1, v2):
                    g.add_edge(v1, v2)
                else:
                    print('Edge already exists.')
 
    elif operation == 'cycle':
        if is_cycle_present(g):
            print('Cycle present.')
        else:
            print('Cycle not present.')
 
    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