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create subcommand-based CLI programs with docopt

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docopt-subcommands

A simple implementation of subcommands for docopt.

docopt-subcommands allows you to create subcommand-based programs using docopt. A subcommand-based program is one in which the main program uses a "subcommand" argument to tell it what to do.

The classic example of such a program is git. The git command with no arguments doesn't do much of anything. Instead, it's the first argument to git - the subcommand - that tells it what to actually do. For example:

git log

will give you the log for your repository. Likewise:

git rebase

is the subcommand for rebasing.

docopt-subcommands is here to help you create similar kinds of command-line tools with docopt'.

Quickstart

The basic idea behind docopt-subcommands is simple:

  1. You provide a separate handler function for each subcommand.
  2. The docstring for each handler function defines the docopt definition for that subcommand.
  3. You register your handler functions with the names of the subcommands which will invoke them.
  4. You provide a program name and (optionally) a top-level documentation string.

Then docopt-subcommands does the work of stitching everything together into a subcommand-driven program. Here's how it looks (from the included exampels/basic_example.py):

# Basic, most common usage of docopt_subcommands

import docopt_subcommands as dsc


# 1. Use the `command` decorator to add subcommands functions.
@dsc.command()
def foo_handler(precommand_args, args):
    """usage: {program} foo <name>

    Apply foo to a name.
    """
    print("Foo, {}".format(args['<name>']))


@dsc.command()
def bar_handler(precommand, args):
    """usage: {program} bar <name>

    Apply bar to a name.
    """
    print("Bar, {}".format(args['<name>']))


# 2. Pass a program name and version string to `main` to run a program with the
# subcommands you defined with the decorators above.
dsc.main(program='docopt-subcommand-example')

If you run this program at the command line you'll see that you have a nice, subcommand-based CLI program:

$ python basic_example.py
Usage: docopt-subcommand-example [options] <command> [<args> ...]
$ python basic_example.py -h
docopt-subcommand-example

Usage: docopt-subcommand-example [options] <command> [<args> ...]

Options:
  -h --help     Show this screen.

Available commands:
  bar
  foo

See 'docopt-subcommand-example help <command>' for help on specific commands.

$ python basic_example.py foo
usage: docopt-subcommand-example foo <name>

$ python basic_example.py foo -h
usage: docopt-subcommand-example foo <name>

Apply foo to a name.

$ python basic_example.py foo Bubba
Foo, Bubba

For more examples, see the examples directory.

Common options

Many subcommand-based programs have a set of options that are common to all commands. A common example is --verbose which causes the program to print more information not matter which command is executed. With docopt_subcommands you specify these common options in the top-level docstring like this::

TOP_LEVEL_DOC = """{program}

Usage: {program} [options] <command> [<args> ...]

Options: -h --help Show this screen --verbose Use verbose output

Available commands: {available_commands} """

With this docstring, you can provide the --verbose flag for any subcommand. Critically, common options must be provided before the subcommand name. So if bar was a subcommand in your program, you would write::

my_program --verbose bar

but not::

my_program bar --verbose

docopt_subcommands parses the complete command line in two passes. The first pass parses it with the top-level docstring while the second pass uses the docstring for the specific command and only parses the part of the command line after the common options. It then provides both of the parsed dicts to the subcommand handler: the first argument to the handler is the result of the first pass, and the second argument is the result of the second pass.

With this system, you can then use a single set of code to do "common configuration" for you program. For example, here's how a handler might look:

.. code-block:: python

@dsc.command() def bar_handler(precommand_args, args): """usage: {program} bar <name>

  Apply bar to a name.
  """
  handle_common_option(precommand_args)
  name = args['<name>']
  print('Bar {}'.format(name))

Advanced usage

For most users the basic usage described in "Quickstart" should be all you need, but some users will need more control of docopt_subcommands. The docopt_subcommands.main() that we used earlier is really just a convenience layer on top of the real workhorse, docopt_subcommands.Subcommands. You can instantiate this class directly, bypassing main(), and interact with it as you need before actually invoke command-line processing.

For the most part, the arguments to the Subcommands initializer are very similar to those to main(). This reflects the fact that main() really just instantiates a Subcommands instance (if you don't provide one), populates it with commands, and calls it with the command line arguments. You can do all of these steps yourself if you need to.

As an example, here's what the basic example above looks like if you construct a Subcommands instance directly.:

import docopt_subcommands as dsc
import sys

sc = dsc.Subcommands(program='docopt-subcommand-example')

@sc.command('foo')
def foo_handler(args):
    """usage: {program} {command} <name>

    Apply foo to a name.
    """
    print("Foo, {}".format(args['<name>']))


@sc.command('bar')
def bar_handler(args):
    """usage: {program} {command} <name>

    Apply bar to a name.
    """
    print("Bar, {}".format(args['<name>']))

sc(sys.argv[1:])

As you can see, it's not substantially different from the basic example. main() primarily just adds a layer of convenience - mostly by choosing reasonable default values for some things - that you lose with this approach.

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