Adding new actions is a bit trickier, because you have to understand that optparse has a couple of classifications for actions:
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 decide if it's a ``store'' action, a ``typed'' action, neither, or both. Three class attributes of Option (or your Option subclass) control this:
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
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.
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