python, How can I put and if to a function that raise an error?

Question

Hi all I am checking for cycle in the graph

import networkx as nx
X2 = {"1": ["4"], "2": ["3"], "3": ["2", "4"], "4": ["1", "3"]}
L2 = []
for k,v in X2.items():
    for i in range(len(v)):
        L2.append((k,v[i]))
print(L2)
G = nx.DiGraph(L2)
G = G.to_undirected()

print(type(G))
print(nx.find_cycle(G))

In this case correctly there aren't cycle so the nx function raise:

raise nx.exception.NetworkXNoCycle('No cycle found.')
networkx.exception.NetworkXNoCycle: No cycle found.

How can I put and If condition to print something if the function raise me the error?

Answer 1

What you are looking for here is error handling. It works like an if for errors like you want.

You achieve this with a try/except block. More details here.

import networkx as nx
X2 = {"1": ["4"], "2": ["3"], "3": ["2", "4"], "4": ["1", "3"]}
L2 = []
for k,v in X2.items():
    for i in range(len(v)):
        L2.append((k,v[i]))
print(L2)
try:
    G = nx.DiGraph(L2)
    G = G.to_undirected()
    print(type(G))
    print(nx.find_cycle(G))
except:
    print("Error message")