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Although it is possible to specify a default value for an append action, 
either directly or through the set_default() function, it is not handled 
correctly when this default is overruled by the user. Take for example 
the following code:
 import optparse
 parser = optparse.OptionParser()
 parser.add_option("-o", "--option", action = "append")
 parser.set_defaults(option = ["a"])
 options, args = parser.parse_args()
 print options
When this is called without arguments the following is printed:
 {'option': ['a']} # as expected
But when it is called with for example with -ob the following is 
printed:
 {'option': ['a', 'b']} # expected {'option': ['b']}
So the default option is also appended, even if the option is explicitly defined by the user.