diff --git a/graphs/basic_graphs.py b/graphs/basic_graphs.py
index ee63ca995de6..1893b26766da 100644
--- a/graphs/basic_graphs.py
+++ b/graphs/basic_graphs.py
@@ -141,7 +141,9 @@ def dijk(G, s):
 from collections import deque
 
 
-def topo(G, ind=None, Q=[1]):
+def topo(G, ind=None, Q=None):
+    if Q is None:
+        Q = [1]
     if ind is None:
         ind = [0] * (len(G) + 1)  # SInce oth Index is ignored
         for u in G: