The attached script demonstrates the bug.

option.dest should be set, even if not explicitly supplied in the call
to parser.add_option, by the time the callback is called.

Unless dest='something' is specified in the call to add_option, you will
get the following error when running "python test.py --myopt":

  File "test.py", line 4, in mycallback
    setattr(parser.values,option.dest,value)
TypeError: attribute name must be string

...as options.dest is still None.

I would expect options.dest to be computed as per the algorithm in
optparse.Option._check_dest.