Extending Optik

Since the two major controlling factors in how Optik interprets command-line options are the action and type of each option, the most likely direction of extension is to add new actions and new types.

Also, the examples/ directory of the source distribution includes several demonstrations of extending Optik in different ways: e.g. a case-insensitive option parser, or two kinds of option parsers that implement "required options".

Adding new types

To add new types, you need to define your own subclass of Optik's Option class. This class has a couple of attributes that define Optik's types: TYPES and TYPE_CHECKER.

TYPES is a tuple of type names; in your subclass, simply define a new tuple TYPES that builds on the standard one.

TYPE_CHECKER is a dictionary mapping type names to type-checking functions. A type-checking function has the following signature:

def check_mytype(option, opt, value)

where option is an Option instance, opt is an option string (e.g., "-f"), and value is the string from the command line that must be checked and converted to your desired type. check_mytype() should return an object of the hypothetical type mytype. The value returned by a type-checking function will wind up in the OptionValues instance returned by OptionParser.parse_args(), or be passed to a callback as the value parameter.

Your type-checking function should raise OptionValueError if it encounters any problems. OptionValueError takes a single string argument, which is passed as-is to OptionParser's error() method, which in turn prepends the program name and the string "error:" and prints everything to stderr before terminating the process.

Here's a silly example that demonstrates adding a complex option type to parse Python-style complex numbers on the command line. (This is even sillier than it used to be, because Optik 1.3 added built-in support for complex numbers, but never mind.)

First, the necessary imports:

from copy import copy
from optik import Option, OptionValueError

You need to define your type-checker first, since it's referred to later (in the TYPE_CHECKER class attribute of your Option subclass):

def check_complex(option, opt, value):
        return complex(value)
    except ValueError:
        raise OptionValueError(
            "option %s: invalid complex value: %r" % (opt, value))

Finally, the Option subclass:

class MyOption (Option):
    TYPES = Option.TYPES + ("complex",)
    TYPE_CHECKER["complex"] = check_complex

(If we didn't make a copy() of Option.TYPE_CHECKER, we would end up modifying the TYPE_CHECKER attribute of Optik's Option class. This being Python, nothing stops you from doing that except good manners and common sense.)

That's it! Now you can write a script that uses the new option type just like any other Optik-based script, except you have to instruct your OptionParser to use MyOption instead of Option:

parser = OptionParser(option_class=MyOption)
parser.add_option("-c", type="complex")

Alternately, you can build your own option list and pass it to OptionParser; if you don't use add_option() in the above way, you don't need to tell OptionParser which option class to use:

option_list = [MyOption("-c", action="store", type="complex", dest="c")]
parser = OptionParser(option_list=option_list)

Adding new actions

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

"store" actions
actions that result in Optik 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 decide if it's a "store" action, a "typed" action, neither, or both. Three class attributes of Option (or your Option subclass) control this:

all actions must be listed in ACTIONS
"store" actions are additionally listed here
"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)
                self, action, dest, opt, value, values, parser)

Features of note:

Other reasons to extend Optik

Adding new types and new actions are the big, obvious reasons why you might want to extend Optik. I can think of at least two other areas to play with.

First, the simple one: OptionParser tries to be helpful by calling sys.exit() when appropriate, i.e. when there's an error on the command line or when the user requests help. In the former case, the traditional course of letting the script crash with a traceback is unacceptable; it will make users think there's a bug in your script when they make a command-line error. In the latter case, there's generally not much point in carrying on after printing a help message.

If this behaviour bothers you, it shouldn't be too hard to "fix" it. You'll have to

  1. subclass OptionParser and override error()
  2. subclass Option and override take_action() -- you'll need to provide your own handling of the help action that doesn't call sys.exit()

The second, much more complex, possibility is to override the command-line syntax implemented by Optik. In this case, you'd leave the whole machinery of option actions and types alone, but rewrite the code that processes sys.argv. You'll need to subclass OptionParser in any case; depending on how radical a rewrite you want, you'll probably need to override one or all of parse_args(), _process_long_opt(), and _process_short_opts().

Both of these are left as an exercise for the reader. I have not tried to implement either myself, since I'm quite happy with Optik's default behaviour (naturally).

Happy hacking, and don't forget: Use the Source, Luke.