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14.3.5.2 Adding new actions

Adding new actions is a bit trickier, because you have to understand that `optparse` has a couple of classifications for actions:

``store'' actions
  actions that result in `optparse` storing a value to an attribute of the current OptionValues instance; these options require a `dest` attribute to be supplied to the Option constructor
``typed'' actions
  actions that take a value from the command line and expect it to be of a certain type; or rather, a string that can be converted to a certain type. These options require a `type` attribute to the Option constructor.

These are overlapping sets: some default ``store'' actions are `store`, `store_const`, `append`, and `count`, while the default ``typed'' actions are `store`, `append`, and `callback`.

When you add an action, you need to categorize it by listing it in at least one of the following class attributes of Option (all are lists of strings):

`ACTIONS`
  all actions must be listed in ACTIONS
`STORE_ACTIONS`
  ``store'' actions are additionally listed here
`TYPED_ACTIONS`
  ``typed'' actions are additionally listed here
`ALWAYS_TYPED_ACTIONS`
  actions that always take a type (i.e. whose options always take a value) are additionally listed here. The only effect of this is that `optparse` assigns the default type, `string`, to options with no explicit type whose action is listed in `ALWAYS_TYPED_ACTIONS`.

In order to actually implement your new action, you must override Option's `take_action()` method and add a case that recognizes your action.

For example, let's add an `extend` action. This is similar to the standard `append` action, but instead of taking a single value from the command-line and appending it to an existing list, `extend` will take multiple values in a single comma-delimited string, and extend an existing list with them. That is, if `"-names"` is an `extend` option of type `string`, the command line

    --names=foo,bar --names blah --names ding,dong

would result in a list

    ["foo", "bar", "blah", "ding", "dong"]

Again we define a subclass of Option:

    class MyOption (Option):

        ACTIONS = Option.ACTIONS + ("extend",)
        STORE_ACTIONS = Option.STORE_ACTIONS + ("extend",)
        TYPED_ACTIONS = Option.TYPED_ACTIONS + ("extend",)
        ALWAYS_TYPED_ACTIONS = Option.ALWAYS_TYPED_ACTIONS + ("extend",)

        def take_action(self, action, dest, opt, value, values, parser):
            if action == "extend":
                lvalue = value.split(",")
                values.ensure_value(dest, []).extend(lvalue)
            else:
                Option.take_action(
                    self, action, dest, opt, value, values, parser)

Features of note:

- `extend` both expects a value on the command-line and stores that value somewhere, so it goes in both `STORE_ACTIONS` and `TYPED_ACTIONS`

- to ensure that `optparse` assigns the default type of `string` to `extend` actions, we put the `extend` action in `ALWAYS_TYPED_ACTIONS` as well

- `MyOption.take_action()` implements just this one new action, and passes control back to `Option.take_action()` for the standard `optparse` actions

- `values` is an instance of the optparse_parser.Values class, which provides the very useful `ensure_value()` method. `ensure_value()` is essentially `getattr()` with a safety valve; it is called as

      values.ensure_value(attr, value)

  If the `attr` attribute of `values` doesn't exist or is None, then ensure_value() first sets it to `value`, and then returns 'value. This is very handy for actions like `extend`, `append`, and `count`, all of which accumulate data in a variable and expect that variable to be of a certain type (a list for the first two, an integer for the latter). Using `ensure_value()` means that scripts using your action don't have to worry about setting a default value for the option destinations in question; they can just leave the default as None and `ensure_value()` will take care of getting it right when it's needed.