# Using usage.Options

## Introduction

There is frequently a need for programs to parse a UNIX-like command line program: options preceded by `-` or `--`, sometimes followed by a parameter, followed by a list of arguments. The `twisted.python.usage` provides a class, `Options`, to facilitate such parsing.

While Python has the `getopt` module for doing this, it provides a very low level of abstraction for options. Twisted has a higher level of abstraction, in the class `twisted.python.usage.Options`. It uses Python's reflection facilities to provide an easy to use yet flexible interface to the command line. While most command line processors either force the application writer to write her own loops, or have arbitrary limitations on the command line (the most common one being not being able to have more then one instance of a specific option, thus rendering the idiom "`program -v -v -v`" impossible), Twisted allows the programmer to decide how much control she wants.

The `Options` class is used by subclassing. Since a lot of time it will be used in the `twisted.tap` package, where the local conventions require the specific options parsing class to also be called `Options`, it is usually imported with

    from twisted.python import usage

## Boolean Options

For simple boolean options, define the attribute `optFlags` like this:

    class Options(usage.Options):

        optFlags = [["fast", "f"], ["safe", "s"]]

`optFlags` should be a list of 2-lists. The first element is the long name, and will be used on the command line as `--fast`. The second one is the short name, and will be used on the command line as `-f`. The long name also determines the name of the key that will be set on the Options instance. Its value will be 1 if the option was seen, 0 otherwise. Here is an example for usage:

    class Options(usage.Options):

        optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]

    command_line = ["-g", "--fast"]

    options = Options()
    try:
        options.parseOptions(command_line)
    except usage.UsageError, errortext:
        print '%s: %s' % (sys.argv[0], errortext)
        print '%s: Try --help for usage details.' % (sys.argv[0])
        sys.exit(1)
    if options['fast']:
        print "fast",
    if options['good']:
        print "good",
    if options['cheap']:
        print "cheap",
    print

The above will print `fast good`.

Note here that Options fully supports the mapping interface. You can access it mostly just like you can access any other dict. Options are stored as mapping items in the Options instance: parameters as 'paramname': 'value' and flags as 'flagname': 1 or 0.

### Inheritance, Or: How I Learned to Stop Worrying and Love the Superclass

Sometimes there is a need for several option processors with a unifying core. Perhaps you want all your commands to understand `-q`/`--quiet` means to be quiet, or something similar. On the face of it, this looks impossible: in Python, the subclass's `optFlags` would shadow the superclass's. However, `usage.Options` uses special reflection code to get all of the `optFlags` defined in the hierarchy. So the following:

    class BaseOptions(usage.Options):

        optFlags = [["quiet", "q"]]

    class SpecificOptions(BaseOptions):

        optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]

Is the same as:

    class SpecificOptions(BaseOptions):

        optFlags = [["quiet", "q"], ["fast", "f"], ["good", "g"], ["cheap", "c"]]

## Parameters

Parameters are specified using the attribute `optParameters`. They must be given a default. If you want to make sure you got the parameter from the command line, give a non-string default. Since the command line only has strings, this is completely reliable.

Here is an example:

    from twisted.python import usage

    class Options(usage.Options):

        optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]
        optParameters = [["user", "u", None]]

    try:
        config.parseOptions() # When given no argument, parses sys.argv[1:]
    except usage.UsageError, errortext:
        print '%s: %s' % (sys.argv[0], errortext)
        print '%s: Try --help for usage details.' % (sys.argv[0])
        sys.exit(1)

    if config['user'] is not None:
        print "Hello," config['user']
    print "So, you want it:"

    if config['fast']:
        print "fast",
    if config['good']:
        print "good",
    if config['cheap']:
        print "cheap",
    print

Like `optFlags`, `optParameters` works smoothly with inheritance.

## Generic Code For Options

Sometimes, just setting an attribute on the basis of the options is not flexible enough. In those cases, Twisted does not even attempt to provide abstractions such as "counts" or "lists", but rathers lets you call your own method, which will be called whenever the option is encountered.

Here is an example of counting verbosity

    from twisted.python import usage

    class Options(usage.Options):

        def __init__(self):
            usage.Options.__init__(self)
    	self['verbosity'] = 0 # default

        def opt_verbose(self):
            self['verbosity'] = self['verbosity']+1

        def opt_quiet(self):
            self['verbosity'] = self['verbosity']-1

        opt_v = opt_verbose
        opt_q = opt_quiet

Command lines that like like "command -v -v -v -v" will increase verbosity to 4, while "command -q -q -q" will decrease verbosity to -3.

The `usage.Options` class knows that these are parameter-less options, since the methods do not receive an argument. Here is an example for a method with a parameter:

    from twisted.python import usage

    class Options(usage.Options):

        def __init__(self):
            usage.Options.__init__(self)
            self['symbols'] = []

        def opt_define(self, symbol):
            self['symbols'].append(symbol)

        opt_D = opt_define

This example is useful for the common idiom of having `command -DFOO -DBAR` to define symbols.

## Parsing Arguments

`usage.Options` does not stop helping when the last parameter is gone. All the other arguments are sent into a function which should deal with them. Here is an example for a `cmp` like command.

    from twisted.python import usage

    class Options(usage.Options):

        optParameters = [["max_differences", "d", 1]]

        def parseArgs(self, origin, changed):
            self['origin'] = origin
            self['changed'] = changed

The command should look like `command origin changed`.

If you want to have a variable number of left-over arguments, just use `def parseArgs(self, *args):`. This is useful for commands like the UNIX `cat(1)`.

## Post Processing

Sometimes, you want to perform post processing of options to patch up inconsistencies, and the like. Here is an example:

    from twisted.python import usage

    class Options(usage.Options):

        optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]

        def postOptions(self):
            if self['fast'] and self['good'] and self['cheap']:
                raise usage.UsageError, "can't have it all, brother"