There are two minor bugs in the example in:
http://docs.python.org/lib/optparse-callback-example-6.html

For the optparse example documentation, the call to parser.add_option is
listed as:

parser.add_option("-c", "--callback",
                  action="callback", callback=varargs)

this has two bugs:
  1. It refers to the wrong function name
  2. It needs to have a dest variable defined, b/c the callback function
operates on that.

This can be fixed if the parser.add_option call is changed to:
parser.add_option("-c", "--callback",
      action="callback", dest="foo", callback=varargs_callback)