6.20.5.2 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 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.

Some default ``store'' actions are store, store_const, append, and count. The default ``typed'' actions are store, append, and callback.

When you add an action, you need to decide if it's a ``store'' action, a ``typed'', neither, or both. Three class attributes of Option (or your Option subclass) control this:

ACTIONS: All actions must be listed as strings in ACTIONS.

STORE_ACTIONS: ``store'' actions are additionally listed here.

TYPED_ACTIONS: ``typed'' actions are additionally listed here.

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",)

    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.
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 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.

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