diff -r e961688ae714 -r 1ca88e24f267 Doc/library/optparse.rst
--- a/Doc/library/optparse.rst	Fri Oct 01 17:40:20 2010 +0200
+++ b/Doc/library/optparse.rst	Mon Oct 04 19:27:05 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, which 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 makes sense if
+  you think that with a default value, :attr:`~Option.dest` is a list containing
+  one element (the default) and any additional value is appended to that list.
+
 * ``"append_const"`` [required: :attr:`~Option.const`; relevant:
   :attr:`~Option.dest`]