Skip to content

# Modular Commands

## Overview

Cmd2 also enables developers to modularize their command definitions into CommandSet objects. CommandSets represent a logical grouping of commands within a `cmd2` application. By default, `CommandSet` objects need to be manually registered. However, it is possible for all `CommandSet` objects to be discovered and loaded automatically when the cmd2.Cmd class is instantiated with this mixin by setting `auto_load_commands=True`. This also enables the developer to dynamically add/remove commands from the `cmd2` application. This could be useful for loadable plugins that add additional capabilities. Additionally, it allows for object-oriented encapsulation and garbage collection of state that is specific to a CommandSet.

### Features

- Modular Command Sets - Commands can be broken into separate modules rather than in one god class holding all commands.
- Automatic Command Discovery - In your application, merely defining and importing a CommandSet is sufficient for `cmd2` to discover and load your command if you set `auto_load_commands=True`. No manual registration is necessary.
- Dynamically Loadable/Unloadable Commands - Command functions and CommandSets can both be loaded and unloaded dynamically during application execution. This can enable features such as dynamically loaded modules that add additional commands.
- Events handlers - Four event handlers are provided in `CommandSet` class for custom initialization and cleanup steps. See Event Handlers.
- Subcommand Injection - Subcommands can be defined separately from the base command. This allows for a more action-centric instead of object-centric command system while still organizing your code and handlers around the objects being managed.

See API documentation for cmd2.CommandSet.

See the examples for more details.

## Defining Commands

### Command Sets

CommandSets group multiple commands together. The plugin will inspect functions within a `CommandSet` using the same rules as when they're defined in `cmd2.Cmd`. Commands must be prefixed with `do_`, help functions with `help_`, and completer functions with `complete_`.

CommandSet command methods will always expect the same parameters as when defined in a `cmd2.Cmd` sub-class, except that `self` will now refer to the `CommandSet` instead of the cmd2 instance. The cmd2 instance can be accessed through `self._cmd` that is populated when the `CommandSet` is registered.

CommandSets will only be auto-loaded if the initializer takes no arguments. If you need to provide initializer arguments, see Manual CommandSet Construction.

    import cmd2
    from cmd2 import CommandSet

    class ExampleApp(cmd2.Cmd):
        """
        CommandSets are automatically loaded. Nothing needs to be done.
        """
        def __init__(self, *args, **kwargs):
            super().__init__(*args, auto_load_commands=True, **kwargs)

        def do_something(self, arg):
            """Something Command."""
            self.poutput('this is the something command')

    class AutoLoadCommandSet(CommandSet[ExampleApp]):
        DEFAULT_CATEGORY = 'My Category'

        def __init__(self):
            super().__init__()

        def do_hello(self, _: cmd2.Statement):
            """Hello Command."""
            self._cmd.poutput('Hello')

        def do_world(self, _: cmd2.Statement):
            """World Command."""
            self._cmd.poutput('World')

### Manual CommandSet Construction

If a CommandSet class requires parameters to be provided to the initializer, you may manually construct CommandSets and pass in the initializer to Cmd2.

    import cmd2
    from cmd2 import CommandSet

    class ExampleApp(cmd2.Cmd):
        """
        CommandSets with initializer parameters are provided in the initializer
        """
        def __init__(self, *args, **kwargs):
            # gotta have this or neither the plugin or cmd2 will initialize
            super().__init__(*args, auto_load_commands=True, **kwargs)

        def do_something(self, arg):
            """Something Command."""
            self.last_result = 5
            self.poutput('this is the something command')

    class CustomInitCommandSet(CommandSet[ExampleApp]):
        DEFAULT_CATEGORY = 'My Category'

        def __init__(self, arg1, arg2):
            super().__init__()

            self._arg1 = arg1
            self._arg2 = arg2

        def do_show_arg1(self, _: cmd2.Statement):
            """Show Arg 1."""
            self._cmd.poutput(f'Arg1: {self._arg1}')

        def do_show_arg2(self, _: cmd2.Statement):
            """Show Arg 2."""
            self._cmd.poutput(f'Arg2: {self._arg2}')

    def main():
        my_commands = CustomInitCommandSet(1, 2)
        app = ExampleApp(command_sets=[my_commands])
        app.cmdloop()

### Type Hinting and self._cmd

When a `CommandSet` is registered, its `_cmd` property is populated with a reference to the `cmd2.Cmd` instance. `CommandSet` is a generic class, allowing you to specify the specific `cmd2.Cmd` subclass it expects to be loaded into.

By parameterizing the inheritance with your application class, your IDE and static analysis tools (like Mypy) will know the exact type of `self._cmd`. This provides full autocompletion and type validation when accessing custom attributes or methods on your main application instance.

    import cmd2
    from cmd2 import CommandSet

    class MyApp(cmd2.Cmd):
        def __init__(self):
            super().__init__()
            self.custom_state = "Some important data"

    class MyCommands(CommandSet[MyApp]):
        def do_check_state(self, _: cmd2.Statement):
            # Type checkers know self._cmd is an instance of MyApp
            # and can validate the 'custom_state' attribute exists.
            self._cmd.poutput(f"State: {self._cmd.custom_state}")

### Dynamic Commands

You can also dynamically load and unload commands by installing and removing CommandSets at runtime. For example, you can support runtime loadable plugins or add/remove commands based on your state.

You may need to disable command auto-loading if you need to dynamically load commands at runtime.

    import argparse
    import cmd2
    from cmd2 import CommandSet, with_argparser, with_category

    class LoadableFruits(CommandSet["ExampleApp"]):
        DEFAULT_CATEGORY = 'Fruits'

        def __init__(self):
            super().__init__()

        def do_apple(self, _: cmd2.Statement):
            """Apple Command."""
            self._cmd.poutput('Apple')

        def do_banana(self, _: cmd2.Statement):
            """Banana Command."""
            self._cmd.poutput('Banana')

    class LoadableVegetables(CommandSet["ExampleApp"]):
        DEFAULT_CATEGORY = 'Vegetables'

        def __init__(self):
            super().__init__()

        def do_arugula(self, _: cmd2.Statement):
            """Arugula Command."""
            self._cmd.poutput('Arugula')

        def do_bokchoy(self, _: cmd2.Statement):
            """Bok Choy Command."""
            self._cmd.poutput('Bok Choy')

    class ExampleApp(cmd2.Cmd):
        """
        CommandSets are loaded via the `load` and `unload` commands
        """

        def __init__(self, *args, **kwargs):
            # gotta have this or neither the plugin or cmd2 will initialize
            super().__init__(*args, auto_load_commands=False, **kwargs)

            self._fruits = LoadableFruits()
            self._vegetables = LoadableVegetables()

        load_parser = cmd2.Cmd2ArgumentParser()
        load_parser.add_argument('cmds', choices=['fruits', 'vegetables'])

        @with_argparser(load_parser)
        @with_category('Command Loading')
        def do_load(self, ns: argparse.Namespace):
            """Load Command."""
            if ns.cmds == 'fruits':
                try:
                    self.register_command_set(self._fruits)
                    self.poutput('Fruits loaded')
                except ValueError:
                    self.poutput('Fruits already loaded')

            if ns.cmds == 'vegetables':
                try:
                    self.register_command_set(self._vegetables)
                    self.poutput('Vegetables loaded')
                except ValueError:
                    self.poutput('Vegetables already loaded')

        @with_argparser(load_parser)
        def do_unload(self, ns: argparse.Namespace):
            """Unload Command."""
            if ns.cmds == 'fruits':
                self.unregister_command_set(self._fruits)
                self.poutput('Fruits unloaded')

            if ns.cmds == 'vegetables':
                self.unregister_command_set(self._vegetables)
                self.poutput('Vegetables unloaded')

    if __name__ == '__main__':
        app = ExampleApp()
        app.cmdloop()

## Event Handlers

The following functions are called at different points in the CommandSet life cycle.

on_register - Called by `cmd2.Cmd` as the first step to registering a `CommandSet`. The commands defined in this class have not be added to the CLI object at this point. Subclasses can override this to perform any initialization requiring access to the Cmd object (e.g. configure commands and their parsers based on CLI state data).

on_registered - Called by `cmd2.Cmd` after a `CommandSet` is registered and all its commands have been added to the CLI. Subclasses can override this to perform custom steps related to the newly added commands (e.g. setting them to a disabled state).

on_unregister - Called by `cmd2.Cmd` as the first step to unregistering a `CommandSet`. Subclasses can override this to perform any cleanup steps which require their commands being registered in the CLI.

on_unregistered - Called by `cmd2.Cmd` after a `CommandSet` has been unregistered and all its commands removed from the CLI. Subclasses can override this to perform remaining cleanup steps.

## Injecting Subcommands

### Description

Using the @with_argparser and @as_subcommand_to decorators, it is possible to easily define subcommands for your command. This has a tendency to drive your interface into an object-centric interface. For example, imagine you have a tool that manages your media collection and you want to manage movies or shows. An object-centric approach would push you to have base commands such as `movies` and `shows` which each have subcommands `add`, `edit`, `list`, `delete`. If you wanted to present an action-centric command set, so that `add`, `edit`, `list`, and `delete` are the base commands, you'd have to organize your code around these similar actions rather than organizing your code around similar objects being managed.

Subcommand injection allows you to inject subcommands into a base command to present an interface that is sensible to a user while still organizing your code in whatever structure makes more logical sense to the developer.

### Example

This example is a variation on the Dynamic Commands example above. A `cut` command is introduced as a base command and each CommandSet adds a subcommand to it.

    import argparse
    import cmd2
    from cmd2 import CommandSet, with_argparser, with_category

    class LoadableFruits(CommandSet["ExampleApp"]):
        DEFAULT_CATEGORY = 'Fruits'

        def __init__(self):
            super().__init__()

        def do_apple(self, _: cmd2.Statement):
            """Apple Command."""
            self._cmd.poutput('Apple')

        banana_parser = cmd2.Cmd2ArgumentParser()
        banana_parser.add_argument('direction', choices=['discs', 'lengthwise'])

        @cmd2.as_subcommand_to('cut', 'banana', banana_parser)
        def cut_banana(self, ns: argparse.Namespace):
            """Cut banana"""
            self._cmd.poutput('cutting banana: ' + ns.direction)

    class LoadableVegetables(CommandSet["ExampleApp"]):
        DEFAULT_CATEGORY = 'Vegetables'

        def __init__(self):
            super().__init__()

        def do_arugula(self, _: cmd2.Statement):
            """Arugula Command."""
            self._cmd.poutput('Arugula')

        bokchoy_parser = cmd2.Cmd2ArgumentParser()
        bokchoy_parser.add_argument('style', choices=['quartered', 'diced'])

        @cmd2.as_subcommand_to('cut', 'bokchoy', bokchoy_parser)
        def cut_bokchoy(self, _: argparse.Namespace):
            """Cut bok choy."""
            self._cmd.poutput('Bok Choy')

    class ExampleApp(cmd2.Cmd):
        """
        CommandSets are loaded dynamically at runtime via other commands.
        """

        def __init__(self, *args, **kwargs):
            # gotta have this or neither the plugin or cmd2 will initialize
            super().__init__(*args, auto_load_commands=False, **kwargs)

            self._fruits = LoadableFruits()
            self._vegetables = LoadableVegetables()

        load_parser = cmd2.Cmd2ArgumentParser()
        load_parser.add_argument('cmds', choices=['fruits', 'vegetables'])

        @with_argparser(load_parser)
        @with_category('Command Loading')
        def do_load(self, ns: argparse.Namespace):
            """Load Command."""
            if ns.cmds == 'fruits':
                try:
                    self.register_command_set(self._fruits)
                    self.poutput('Fruits loaded')
                except ValueError:
                    self.poutput('Fruits already loaded')

            if ns.cmds == 'vegetables':
                try:
                    self.register_command_set(self._vegetables)
                    self.poutput('Vegetables loaded')
                except ValueError:
                    self.poutput('Vegetables already loaded')

        @with_argparser(load_parser)
        def do_unload(self, ns: argparse.Namespace):
            """Unload Command."""
            if ns.cmds == 'fruits':
                self.unregister_command_set(self._fruits)
                self.poutput('Fruits unloaded')

            if ns.cmds == 'vegetables':
                self.unregister_command_set(self._vegetables)
                self.poutput('Vegetables unloaded')

        cut_parser = cmd2.Cmd2ArgumentParser()
        cut_parser.add_subparsers(title="item", help="item to cut", metavar="ITEM", required=True)

        @with_argparser(cut_parser)
        def do_cut(self, ns: argparse.Namespace):
            """Cut Command."""
            # Call whatever subcommand function was selected
            ns.cmd2_subcommand_func(ns)

    if __name__ == '__main__':
        app = ExampleApp()
        app.cmdloop()