diff --git a/whats_left.py b/whats_left.py
index 8a8f807ca..ec8bd4dcd 100755
--- a/whats_left.py
+++ b/whats_left.py
@@ -25,6 +25,11 @@ import subprocess
 import platform
 from pydoc import ModuleScanner
 
+if not sys.flags.isolated:
+    print("running without -I option.")
+    print("python -I whats_left.py")
+    exit(1)
+
 GENERATED_FILE = "extra_tests/snippets/not_impl.py"
 
 implementation = platform.python_implementation()