diff --git a/tests/io/write_ext.py b/tests/io/write_ext.py
index f4ed0d3685..19b616174b 100644
--- a/tests/io/write_ext.py
+++ b/tests/io/write_ext.py
@@ -1,3 +1,5 @@
+# This tests extended (MicroPython-specific) form of write:
+# write(buf, len) and write(buf, offset, len)
 import uio
 
 try: