Detect cycle in an undirected graph (python)

union_find.py

#python3

# It's simpler version of the example posted at:
# https://www.geeksforgeeks.org/union-find/

class Graph:    
    edges = set()
   
    def add_edge(self, src, dst):
        '''Each edge has 2 vertices: source and destination.'''
        self.edges.add( (src,dst) )

    def get_root(self, vertex):
        '''Recursive function to get root vertex.'''
        if self.parents[vertex] != -1:
            return self.get_root(self.parents[vertex])
        return vertex
        
    def is_cyclic(self):
        self.parents = dict()
        
        # init parents of all vertices to -1
        for src, dst in self.edges:
            self.parents[src] = -1 
            self.parents[dst] = -1

        # src and dst are vertices, e.g. src='a', dst='b'
        for src, dst in self.edges:                
            x = self.get_root(src)
            y = self.get_root(dst)
            if x == y:
                return True
            
            self.parents[x] = y


''' Graph visualised:

a - b - c - d - e
    |   |
V - W - X - Y - Z

'''
                
g = Graph()
# 1st separate "line" of vertices connected together (marked with lower-case).
g.add_edge('a', 'b')
g.add_edge('b', 'c')
g.add_edge('c', 'd')
g.add_edge('d', 'e')

# 2nd separate "line" of vertices connected together (marked with upper-case).
g.add_edge('V', 'W')
g.add_edge('W', 'X')
g.add_edge('X', 'Y')
g.add_edge('Y', 'Z')

# Bridge between these 2 lines (forming closed cycle).
g.add_edge('b', 'W')
g.add_edge('c', 'X')
# Commenting-out 1 of the lines above should result
# in "Graph does not contain cycle" message.

if g.is_cyclic(): 
    print ("Graph contains cycle")
else : 
    print ("Graph does not contain cycle ")