diff --git a/tests/runtests.py b/tests/runtests.py
index 50744122e9..b71cf98b13 100755
--- a/tests/runtests.py
+++ b/tests/runtests.py
@@ -61,6 +61,7 @@ def get_test_modules():
         for f in os.listdir(dirpath):
             if (f.startswith('__init__') or
                 f.startswith('.') or
+                # Python 3 byte code dirs (PEP 3147)
                 f == '__pycache__' or
                 f.startswith('sql') or
                 os.path.basename(f) in REGRESSION_SUBDIRS_TO_SKIP):