cmd2 Plugin Template

Table of Contents

• Using this template
• Naming
• Adding functionality
• Examples
• Development Tasks
• Packaging and Distribution
• License

Using this template

This template assumes you are creating a new cmd2 plugin called myplugin. Your plugin will have a different name. You will need to rename some of the files and directories in this template. Don't forget to modify the imports and setup.py.

You'll probably also want to rewrite the README :)

Naming

You should prefix the name of your project with cmd2-. Within that project, you should have a package with a prefix of cmd2_.

Adding functionality

There are many ways to add functionality to cmd2 using a plugin. Most plugins will be implemented as a mixin. A mixin is a class that encapsulates and injects code into another class. Developers who use a plugin in their cmd2 project, will inject the plugin's code into their subclass of cmd2.Cmd.

Mixin and Initialization

The following short example shows how to mix in a plugin and how the plugin gets initialized.

Here's the plugin:

class MyPlugin:
    def __init__(self, *args, **kwargs):
        # code placed here runs before cmd2.Cmd initializes
        super().__init__(*args, **kwargs)
        # code placed here runs after cmd2.Cmd initializes

and an example app which uses the plugin:

import cmd2
import cmd2_myplugin

class Example(cmd2_myplugin.MyPlugin, cmd2.Cmd):
    """An class to show how to use a plugin"""
    def __init__(self, *args, **kwargs):
        # code placed here runs before cmd2.Cmd or
        # any plugins initialize
        super().__init__(*args, **kwargs)
        # code placed here runs after cmd2.Cmd and
        # all plugins have initialized

Note how the plugin must be inherited (or mixed in) before cmd2.Cmd. This is required for two reasons:

• The cmd.Cmd.__init__() method in the python standard library does not call super().__init__(). Because of this oversight, if you don't inherit from MyPlugin first, the MyPlugin.__init__() method will never be called.
• You may want your plugin to be able to override methods from cmd2.Cmd. If you mixin the plugin after cmd2.Cmd, the python method resolution order will call cmd2.Cmd methods before it calls those in your plugin.

Add commands

Your plugin can add user visable commands. You do it the same way in a plugin that you would in a cmd2.Cmd app:

class MyPlugin:

    def do_say(self, statement):
        """Simple say command"""
        self.poutput(statement)

You have all the same capabilities within the plugin that you do inside a cmd2.Cmd app, including argument parsing via decorators and custom help methods.

Add (or hide) settings

A plugin may add user controllable settings to the application. Here's an example:

class MyPlugin:
    def __init__(self, *args, **kwargs):
        # code placed here runs before cmd2.Cmd initializes
        super().__init__(*args, **kwargs)
        # code placed here runs after cmd2.Cmd initializes
        self.mysetting = 'somevalue'
        self.settable.update({'mysetting': 'short help message for mysetting'})

You can also hide settings from the user by removing them from self.settable.

Decorators

Your plugin can provide a decorator which users of your plugin can use to wrap functionality around their own commands.

Override methods

Your plugin can override core cmd2.Cmd methods, changing their behavior. This approach should be used sparingly, because it is very brittle. If a developer chooses to use multiple plugins in their application, and several of the plugins override the same method, only the first plugin to be mixed in will have the overridden method called.

Hooks are a much better approach.

Hooks

Plugins can register hooks, which are called by cmd2.Cmd during various points in the application and command processing lifecycle. Plugins should not override any of the deprecated hook methods, instead they should register their hooks as described in the cmd2 documentation.

You should name your hooks so that they begin with the name of your plugin. Hook methods get mixed into the cmd2 application and this naming convention helps avoid unintentional method overriding.

Here's a simple example:

class MyPlugin:

    def __init__(self, *args, **kwargs):
        # code placed here runs before cmd2 initializes
        super().__init__(*args, **kwargs)
        # code placed here runs after cmd2 initializes
        # this is where you register any hook functions
        self.register_postparsing_hook(self.cmd2_myplugin_postparsing_hook)

    def cmd2_myplugin_postparsing_hook(self, data: cmd2.plugin.PostparsingData) -> cmd2.plugin.PostparsingData:
        """Method to be called after parsing user input, but before running the command"""
        self.poutput('in postparsing_hook')
        return data

Registration allows multiple plugins (or even the application itself) to each inject code to be called during the application or command processing lifecycle.

See the cmd2 hook documentation for full details of the application and command lifecycle, including all available hooks and the ways hooks can influence the lifecycle.

Classes and Functions

Your plugin can also provide classes and functions which can be used by developers of cmd2 based applications. Describe these classes and functions in your documentation so users of your plugin will know what's available.

Examples

Include an example or two in the examples directory which demonstrate how your plugin works. This will help developers utilize it from within their application.

Development Tasks

This project uses many other python modules for various development tasks, including testing, linting, building wheels, and distributing releases. These modules can be configured many different ways, which can make it difficult to learn the specific incantations required for each project you are familiar with.

This project uses invoke to provide a clean, high level interface for these development tasks. To see the full list of functions available:

$ invoke -l

You can run multiple tasks in a single invocation, for example:

$ invoke clean docs sdist wheel

That one command will remove all superflous cache, testing, and build files, render the documentation, and build a source distribution and a wheel distribution.

For more information, read tasks.py.

While developing your plugin, you should make sure you support all versions of python supported by cmd2, and all supported platforms. cmd2 uses a three tiered testing strategy to accomplish this objective.

• pytest runs the unit tests
• tox runs the unit tests on multiple versions of python
• AppVeyor and TravisCI run the tests on the various supported platforms

This plugin template is set up to use the same strategy.

Create python environments

This project uses tox to run the test suite against multiple python versions. I recommend pyenv with the pyenv-virtualenv plugin to manage these various versions. If you are a Windows user, pyenv won't work for you, but conda can also be used to solve this problem.

This distribution includes a shell script build-pyenvs.sh which automates the creation of these environments.

If you prefer to create these virtualenvs by hand, do the following:

$ cd cmd2_abbrev
$ pyenv install 3.7.0
$ pyenv virtualenv -p python3.7 3.7.0 cmd2-3.7
$ pyenv install 3.6.5
$ pyenv virtualenv -p python3.6 3.6.5 cmd2-3.6
$ pyenv install 3.5.5
$ pyenv virtualenv -p python3.5 3.5.5 cmd2-3.5
$ pyenv install 3.4.8
$ pyenv virtualenv -p python3.4 3.4.8 cmd2-3.4

Now set pyenv to make all three of those available at the same time:

$ pyenv local cmd2-3.7 cmd2-3.6 cmd2-3.5 cmd2-3.4

Whether you ran the script, or did it by hand, you now have isolated virtualenvs for each of the major python versions. This table shows various python commands, the version of python which will be executed, and the virtualenv it will utilize.

Command	python	virtualenv
python	3.7.0	cmd2-3.6
python3	3.7.0	cmd2-3.6
python3.7	3.7.0	cmd2-3.7
python3.6	3.6.5	cmd2-3.6
python3.5	3.5.5	cmd2-3.5
python3.4	3.4.8	cmd2-3.4
pip	3.7.0	cmd2-3.6
pip3	3.7.0	cmd2-3.6
pip3.7	3.7.0	cmd2-3.7
pip3.6	3.6.5	cmd2-3.6
pip3.5	3.5.5	cmd2-3.5
pip3.4	3.4.8	cmd2-3.4

Install Dependencies

Install all the development dependencies:

$ pip install -e .[dev]

This command also installs cmd2-myplugin "in-place", so the package points to the source code instead of copying files to the python site-packages folder.

All the dependencies now have been installed in the cmd2-3.7 virtualenv. If you want to work in other virtualenvs, you'll need to manually select it, and install again::

$ pyenv shell cmd2-3.4 $ pip install -e .[dev]

Now that you have your python environments created, you need to install the package in place, along with all the other development dependencies:

$ pip install -e .[dev]

Running unit tests

Run invoke pytest from the top level directory of your plugin to run all the unit tests found in the tests directory.

Use tox to run unit tests in multiple versions of python

The included tox.ini is setup to run the unit tests in python 3.4, 3.5, 3.6, and 3.7. You can run your unit tests in all of these versions of python by:

$ invoke tox

Run unit tests on multiple platforms

AppVeyor and TravisCI offer free plans for open source projects.

Packaging and Distribution

When creating your setup.py file, keep the following in mind:

• use the keywords cmd2 plugin to make it easier for people to find your plugin
• since cmd2 uses semantic versioning, you should use something like install_requires=['cmd2 >= 0.9.4, <=2'] to make sure that your plugin doesn't try and run with a future version of cmd2 with which it may not be compatible

License

cmd2 uses the very liberal MIT license. We invite plugin authors to consider doing the same.