diff --git a/Lib/commands.py b/Lib/commands.py
index 428a830764f..6471a8db594 100644
--- a/Lib/commands.py
+++ b/Lib/commands.py
@@ -40,6 +40,8 @@ def mk2arg(head, x):
 
 
 # Make a shell command argument from a string.
+# Return a string beginning with a space followed by a shell-quoted
+# version of the argument.
 # Two strategies: enclose in single quotes if it contains none;
 # otherwise, enclose in double quotes and prefix quotable characters
 # with backslash.