Flag options--set a variable to true or false when a particular option
is seen--are quite common. optparse supports them with two separate
actions, store_true and store_false. For example, you might have a
verbose flag that is turned on with "-v" and off with "-q":
parser.add_option("-v", action="store_true", dest="verbose")
parser.add_option("-q", action="store_false", dest="verbose")
Here we have two different options with the same destination, which is perfectly OK. (It just means you have to be a bit careful when setting default values--see below.)
When optparse encounters "-v" on the command line, it sets
options.verbose to True; when it encounters "-q",
options.verbose is set to False.
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