diff -r e961688ae714 -r 8d612ccff747 Doc/library/optparse.rst
--- a/Doc/library/optparse.rst	Fri Oct 01 17:40:20 2010 +0200
+++ b/Doc/library/optparse.rst	Fri Oct 01 20:17:25 2010 +0200
@@ -1088,6 +1088,14 @@
 
      options.tracks.append(int("4"))
 
+  An interesting behaviour to note is what happens when there is a default
+  value: any option passed on the command-line is `appended` to the
+  :attr:`~Option.dest` variable, that already contains the default value; it
+  means that the default value is preserved in case of additional options and
+  not replaced (contrary to what one can think). This behaviour is clear if you
+  think that with a default value :attr:`~Option.dest` is a list containing one
+  element (the default) and any additional option is appended to that list.
+
 * ``"append_const"`` [required: :attr:`~Option.const`; relevant:
   :attr:`~Option.dest`]