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# Extension development

Mopidy started as simply an MPD server that could play music from Spotify. Early on, Mopidy got multiple “frontends” to expose Mopidy to more than just MPD clients: for example the scrobbler frontend that scrobbles your listening history to your Last.fm account, the MPRIS frontend that integrates Mopidy into the Ubuntu Sound Menu, and the HTTP server and JavaScript player API making web based Mopidy clients possible. In Mopidy 0.9 we added support for multiple music sources without stopping and reconfiguring Mopidy: for example the local backend for playing music from your disk, the stream backend for playing Internet radio streams, and the Spotify and SoundCloud backends, for playing music directly from those services.

All of these are examples of what you can accomplish by creating a Mopidy extension. If you want to create your own Mopidy extension for something that does not exist yet, this guide to extension development will help you get your extension running in no time, and make it feel the way users would expect your extension to behave.

## Anatomy of an extension

Extensions are located in a Python package called `mopidy_something` where “something” is the name of the application, library or web service you want to integrate with Mopidy. So, for example, if you plan to add support for a service named Soundspot to Mopidy, you would name your extension’s Python package `mopidy_soundspot`.

The extension must be shipped with a `setup.py` file and be registered on PyPI. The name of the distribution on PyPI would be something like “Mopidy-Soundspot”. Make sure to include the name “Mopidy” somewhere in that name and that you check the capitalization. This is the name users will use when they install your extension from PyPI.

Mopidy extensions must be licensed under an Apache 2.0 (like Mopidy itself), BSD, MIT or more liberal license to be able to be enlisted in the Mopidy documentation. The license text should be included in the `LICENSE` file in the root of the extension’s Git repo.

Combining this together, we get the following folder structure for our extension, Mopidy-Soundspot:

    mopidy-soundspot/           # The Git repo root
        LICENSE                 # The license text
        MANIFEST.in             # List of data files to include in PyPI package
        README.rst              # Document what it is and how to use it
        mopidy_soundspot/       # Your code
            __init__.py
            ext.conf            # Default config for the extension
            ...
        setup.py                # Installation script

Example content for the most important files follows below.

## cookiecutter project template

We’ve also made a cookiecutter project template for creating new Mopidy extensions. If you install cookiecutter and run a single command, you’re asked a few questions about the name of your extension, etc. This is used to create a folder structure similar to the above, with all the needed files and most of the details filled in for you. This saves you a lot of tedious work and copy-pasting from this howto. See the readme of cookiecutter-mopidy-ext for further details.

## Example README.rst

The README file should quickly explain what the extension does, how to install it, and how to configure it. It should also contain a link to a tarball of the latest development version of the extension. It’s important that this link ends with `#egg=Mopidy-Something-dev` for installation using `pip install Mopidy-Something==dev` to work.

    ****************
    Mopidy-Soundspot
    ****************

    `Mopidy <http://www.mopidy.com/>`_ extension for playing music from
    `Soundspot <http://soundspot.example.com/>`_.

    Requires a Soundspot Platina subscription and the pysoundspot library.

    Installation
    ============

    Install by running::

        sudo pip install Mopidy-Soundspot

    Or, if available, install the Debian/Ubuntu package from `apt.mopidy.com
    <http://apt.mopidy.com/>`_.

    Configuration
    =============

    Before starting Mopidy, you must add your Soundspot username and password
    to the Mopidy configuration file::

        [soundspot]
        username = alice
        password = secret

    Project resources
    =================

    - `Source code <https://github.com/mopidy/mopidy-soundspot>`_
    - `Issue tracker <https://github.com/mopidy/mopidy-soundspot/issues>`_
    - `Development branch tarball <https://github.com/mopidy/mopidy-soundspot/tarball/master#egg=Mopidy-Soundspot-dev>`_

    Changelog
    =========

    v0.1.0 (2013-09-17)
    -------------------

    - Initial release.

## Example setup.py

The `setup.py` file must use setuptools, and not distutils. This is because Mopidy extensions use setuptools’ entry point functionality to register themselves as available Mopidy extensions when they are installed on your system.

The example below also includes a couple of convenient tricks for reading the package version from the source code so that it is defined in a single place, and to reuse the README file as the long description of the package for the PyPI registration.

The package must have `install_requires` on `setuptools` and `Mopidy >= 0.14` (or a newer version, if your extension requires it), in addition to any other dependencies required by your extension. If you implement a Mopidy frontend or backend, you’ll need to include `Pykka >= 1.1` in the requirements. The `entry_points` part must be included. The `mopidy.ext` part cannot be changed, but the innermost string should be changed. It’s format is `ext_name = package_name:Extension`. `ext_name` should be a short name for your extension, typically the part after “Mopidy-” in lowercase. This name is used e.g. to name the config section for your extension. The `package_name:Extension` part is simply the Python path to the extension class that will connect the rest of the dots.

    from __future__ import absolute_import, unicode_literals

    import re
    from setuptools import setup, find_packages

    def get_version(filename):
        content = open(filename).read()
        metadata = dict(re.findall("__([a-z]+)__ = '([^']+)'", content))
        return metadata['version']

    setup(
        name='Mopidy-Soundspot',
        version=get_version('mopidy_soundspot/__init__.py'),
        url='https://github.com/your-account/mopidy-soundspot',
        license='Apache License, Version 2.0',
        author='Your Name',
        author_email='your-email@example.com',
        description='Very short description',
        long_description=open('README.rst').read(),
        packages=find_packages(exclude=['tests', 'tests.*']),
        zip_safe=False,
        include_package_data=True,
        install_requires=[
            'setuptools',
            'Mopidy >= 0.14',
            'Pykka >= 1.1',
            'pysoundspot',
        ],
        entry_points={
            'mopidy.ext': [
                'soundspot = mopidy_soundspot:Extension',
            ],
        },
        classifiers=[
            'Environment :: No Input/Output (Daemon)',
            'Intended Audience :: End Users/Desktop',
            'License :: OSI Approved :: Apache Software License',
            'Operating System :: OS Independent',
            'Programming Language :: Python :: 2',
            'Topic :: Multimedia :: Sound/Audio :: Players',
        ],
    )

To make sure your README, license file and default config file is included in the package that is uploaded to PyPI, we’ll also need to add a `MANIFEST.in` file:

    include LICENSE
    include MANIFEST.in
    include README.rst
    include mopidy_soundspot/ext.conf

For details on the `MANIFEST.in` file format, check out the distutils docs. check-manifest is a very useful tool to check your `MANIFEST.in` file for completeness.

## Example __init__.py

The `__init__.py` file should be placed inside the `mopidy_soundspot` Python package.

The root of your Python package should have an `__version__` attribute with a PEP 386 compliant version number, for example “0.1”. Next, it should have a class named `Extension` which inherits from Mopidy’s extension base class, `mopidy.ext.Extension`. This is the class referred to in the `entry_points` part of `setup.py`. Any imports of other files in your extension, outside of Mopidy and it’s core requirements, should be kept inside methods. This ensures that this file can be imported without raising `ImportError` exceptions for missing dependencies, etc.

The default configuration for the extension is defined by the `get_default_config()` method in the `Extension` class which returns a `ConfigParser` compatible config section. The config section’s name must be the same as the extension’s short name, as defined in the `entry_points` part of `setup.py`, for example `soundspot`. All extensions must include an `enabled` config which normally should default to `true`. Provide good defaults for all config values so that as few users as possible will need to change them. The exception is if the config value has security implications; in that case you should default to the most secure configuration. Leave any configurations that don’t have meaningful defaults blank, like `username` and `password`. In the example below, we’ve chosen to maintain the default config as a separate file named `ext.conf`. This makes it easy to include the default config in documentation without duplicating it.

This is `mopidy_soundspot/__init__.py`:

    from __future__ import absolute_import, unicode_literals

    import logging
    import os

    import pygst
    pygst.require('0.10')
    import gst
    import gobject

    from mopidy import config, exceptions, ext

    __version__ = '0.1'

    # If you need to log, use loggers named after the current Python module
    logger = logging.getLogger(__name__)

    class Extension(ext.Extension):

        dist_name = 'Mopidy-Soundspot'
        ext_name = 'soundspot'
        version = __version__

        def get_default_config(self):
            conf_file = os.path.join(os.path.dirname(__file__), 'ext.conf')
            return config.read(conf_file)

        def get_config_schema(self):
            schema = super(Extension, self).get_config_schema()
            schema['username'] = config.String()
            schema['password'] = config.Secret()
            return schema

        def get_command(self):
            from .commands import SoundspotCommand
            return SoundspotCommand()

        def validate_environment(self):
            # Any manual checks of the environment to fail early.
            # Dependencies described by setup.py are checked by Mopidy, so you
            # should not check their presence here.
            pass

        def setup(self, registry):
            # You will typically only do one of the following things in a
            # single extension.

            # Register a frontend
            from .frontend import SoundspotFrontend
            registry.add('frontend', SoundspotFrontend)

            # Register a backend
            from .backend import SoundspotBackend
            registry.add('backend', SoundspotBackend)

            # Or nothing to register e.g. command extension
            pass

And this is `mopidy_soundspot/ext.conf`:

    [soundspot]
    enabled = true
    username =
    password =

For more detailed documentation on the extension class, see the Extension API.

## Example frontend

If you want to use Mopidy’s core API from your extension, then you want to implement a frontend.

The skeleton of a frontend would look like this. Notice that the frontend gets passed a reference to the core API when it’s created. See the Frontend API for more details.

    import pykka

    from mopidy import core

    class SoundspotFrontend(pykka.ThreadingActor, core.CoreListener):
        def __init__(self, config, core):
            super(SoundspotFrontend, self).__init__()
            self.core = core

        # Your frontend implementation

## Example backend

If you want to extend Mopidy to support new music and playlist sources, you want to implement a backend. A backend does not have access to Mopidy’s core API at all, but it does have a bunch of interfaces it can implement to extend Mopidy.

The skeleton of a backend would look like this. See Backend API for more details.

    import pykka

    from mopidy import backend

    class SoundspotBackend(pykka.ThreadingActor, backend.Backend):
        def __init__(self, config, audio):
            super(SoundspotBackend, self).__init__()
            self.audio = audio

        # Your backend implementation

## Example command

If you want to extend the Mopidy with a new helper not run from the server, such as scanning for media, adding a command is the way to go. Your top level command name will always match your extension name, but you are free to add sub-commands with names of your choosing.

The skeleton of a command would look like this. See Commands API for more details.

    from mopidy import commands

    class SoundspotCommand(commands.Command):
        help = 'Some text that will show up in --help'

        def __init__(self):
            super(SoundspotCommand, self).__init__()
            self.add_argument('--foo')

        def run(self, args, config, extensions):
           # Your command implementation
           return 0

## Example web application

As of Mopidy 0.19, extensions can use Mopidy’s built-in web server to host static web clients as well as Tornado and WSGI web applications. For several examples, see the HTTP server side API docs or explore with Mopidy-API-Explorer extension.

## Running an extension

Once your extension is ready to go, to see it in action you’ll need to register it with Mopidy. Typically this is done by running `python setup.py install` from your extension’s Git repo root directory. While developing your extension and to avoid doing this every time you make a change, you can instead run `python setup.py develop` to effectively link Mopidy directly with your development files.

## Python conventions

In general, it would be nice if Mopidy extensions followed the same Code style as Mopidy itself, as they’re part of the same ecosystem. Among other things, the code style guide explains why all the above examples start with `from __future__ import absolute_import, unicode_literals`.

## Use of Mopidy APIs

When writing an extension, you should only use APIs documented at API reference. Other parts of Mopidy, like `mopidy.utils`, may change at any time and are not something extensions should use.

## Logging in extensions

For servers like Mopidy, logging is essential for understanding what’s going on. We use the `logging` module from Python’s standard library. When creating a logger, always namespace the logger using your Python package name as this will be visible in Mopidy’s debug log:

    import logging

    logger = logging.getLogger('mopidy_soundspot')

    # Or even better, use the Python module name as the logger name:
    logger = logging.getLogger(__name__)

When logging at logging level `info` or higher (i.e. `warning`, `error`, and `critical`, but not `debug`) the log message will be displayed to all Mopidy users. Thus, the log messages at those levels should be well written and easy to understand.

As the logger name is not included in Mopidy’s default logging format, you should make it obvious from the log message who is the source of the log message. For example:

    Loaded 17 Soundspot playlists

Is much better than:

    Loaded 17 playlists

If you want to turn on debug logging for your own extension, but not for everything else due to the amount of noise, see the docs for the `loglevels/*` config section.

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