diff --git a/ch9/apps.py b/ch9/apps.py
new file mode 100644
index 0000000..37990d7
--- /dev/null
+++ b/ch9/apps.py
@@ -0,0 +1,32 @@
+def buggy(arg, result=[]):
+    result.append(arg)
+    return result
+
+
+def works(arg):
+    result = []
+    result.append(arg)
+    return result
+
+
+def nonbuggy(arg, result=[]):
+    if result is None:
+        result = []
+    result.append(arg)
+    return result
+
+
+print(buggy(5))
+print(buggy(6))
+print(buggy(7))
+print(buggy(8))
+print()
+print(works(5))
+print(works(6))
+print(works(7))
+print(works(8))
+print()
+print(nonbuggy(5))
+print(nonbuggy(6))
+print(nonbuggy(7))
+print(nonbuggy(8))