# 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 <https://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
    <https://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.

    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`:

    import logging
    import os

    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 mopidy.ext – 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 mopidy.backend — 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 mopidy.commands — 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 the 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.

## Use of Mopidy APIs

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

Mopidy performs type checking to help catch extension bugs. This applies to both frontend calls into core and return values from backends. Additionally model fields always get validated to further guard against bad data.

## 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.

## Making HTTP requests from extensions

Many Mopidy extensions need to make HTTP requests to use some web API. Here’s a few recommendations to those extensions.

### Proxies

If you make HTTP requests please make sure to respect the proxy configs, so that all the requests you make go through the proxy configured by the Mopidy user. To make this easier for extension developers, the helper function `mopidy.httpclient.format_proxy()` was added in Mopidy 1.1. This function returns the proxy settings formatted the way Requests expects.

### User-Agent strings

When you make HTTP requests, it’s helpful for debugging and usage analysis if the client identifies itself with a proper User-Agent string. In Mopidy 1.1, we added the helper function `mopidy.httpclient.format_user_agent()`. Here’s an example of how to use it:

    >>> from mopidy import httpclient
    >>> import mopidy_soundspot
    >>> httpclient.format_user_agent(
    ...     f'{mopidy_soundspot.Extension.dist_name}/'
    ...     f'{mopidy_soundspot.__version__}'
    ... )
    'Mopidy-SoundSpot/2.0.0 Mopidy/3.0.0 Python/3.9.2'

### Example using Requests sessions

Most Mopidy extensions that make HTTP requests use the Requests library to do so. When using Requests, the most convenient way to make sure the proxy and User-Agent header is set properly is to create a Requests session object and use that object to make all your HTTP requests:

    from mopidy import httpclient

    import requests

    import mopidy_soundspot

    def get_requests_session(proxy_config, user_agent):
        proxy = httpclient.format_proxy(proxy_config)
        full_user_agent = httpclient.format_user_agent(user_agent)

        session = requests.Session()
        session.proxies.update({'http': proxy, 'https': proxy})
        session.headers.update({'user-agent': full_user_agent})

        return session

    # ``mopidy_config`` is the config object passed to your frontend/backend
    # constructor
    session = get_requests_session(
        proxy_config=mopidy_config['proxy'],
        user_agent=(
            f'{mopidy_soundspot.Extension.dist_name}/{mopidy_soundspot.__version__}'
        )
    )

    response = session.get('https://example.com')
    # Now do something with ``response`` and/or make further requests using the
    # ``session`` object.

For further details, see Requests’ docs on session objects.

## Testing extensions

Creating test cases for your extensions makes them much simpler to maintain over the long term. It can also make it easier for you to review and accept pull requests from other contributors knowing that they will not break the extension in some unanticipated way.

Before getting started, it is important to familiarize yourself with the Python mock library. When it comes to running tests, Mopidy typically makes use of testing tools like tox and pytest.

### Testing approach

To a large extent the testing approach to follow depends on how your extension is structured, which parts of Mopidy it interacts with, and if it uses any 3rd party APIs or makes any HTTP requests to the outside world.

The sections that follow contain code extracts that highlight some of the key areas that should be tested. For more exhaustive examples, you may want to take a look at the test cases that ship with Mopidy itself which covers everything from instantiating various controllers, reading configuration files, and simulating events that your extension can listen to.

In general your tests should cover the extension definition, the relevant Mopidy controllers, and the Pykka backend and / or frontend actors that form part of the extension.

### Testing the extension definition

Test cases for checking the definition of the extension should ensure that:

- the extension provides a `ext.conf` configuration file containing the relevant parameters with their default values,

- that the config schema is fully defined, and

- that the extension’s actor(s) are added to the Mopidy registry on setup.

An example of what these tests could look like is provided below:

    def test_get_default_config():
        ext = Extension()
        config = ext.get_default_config()

        assert '[my_extension]' in config
        assert 'enabled = true' in config
        assert 'param_1 = value_1' in config
        assert 'param_2 = value_2' in config
        assert 'param_n = value_n' in config

    def test_get_config_schema():
        ext = Extension()
        schema = ext.get_config_schema()

        assert 'enabled' in schema
        assert 'param_1' in schema
        assert 'param_2' in schema
        assert 'param_n' in schema

    def test_setup():
        registry = mock.Mock()

        ext = Extension()
        ext.setup(registry)
        calls = [mock.call('frontend', frontend_lib.MyFrontend),
                 mock.call('backend',  backend_lib.MyBackend)]
        registry.add.assert_has_calls(calls, any_order=True)

### Testing backend actors

Backends can usually be constructed with a small mockup of the configuration file, and mocking the audio actor:

    @pytest.fixture
    def config():
        return {
            'http': {
                'hostname': '127.0.0.1',
                'port': '6680'
            },
            'proxy': {
                'hostname': 'host_mock',
                'port': 'port_mock'
            },
            'my_extension': {
                'enabled': True,
                'param_1': 'value_1',
                'param_2': 'value_2',
                'param_n': 'value_n',
            }
        }

    def get_backend(config):
        return backend.MyBackend(config=config, audio=mock.Mock())

The following libraries might be useful for mocking any HTTP requests that your extension makes:

- responses - A utility library for mocking out the requests Python library.

- vcrpy - Automatically mock your HTTP interactions to simplify and speed up testing.

At the very least, you’ll probably want to patch `requests` or any other web API’s that you use to avoid any unintended HTTP requests from being made by your backend during testing:

    from mock import patch
    @mock.patch('requests.get',
                mock.Mock(side_effect=Exception('Intercepted unintended HTTP call')))

Backend tests should also ensure that:

- the backend provides a unique URI scheme,

- that it sets up the various providers (e.g. library, playback, etc.)

    def test_uri_schemes(config):
        backend = get_backend(config)

        assert 'my_scheme' in backend.uri_schemes

    def test_init_sets_up_the_providers(config):
        backend = get_backend(config)

        assert isinstance(backend.library, library.MyLibraryProvider)
        assert isinstance(backend.playback, playback.MyPlaybackProvider)

Once you have a backend instance to work with, testing the various playback, library, and other providers is straight forward and should not require any special setup or processing.

### Testing libraries

Library test cases should cover the implementations of the standard Mopidy API (e.g. `browse`, `lookup`, `refresh`, `get_images`, `search`, etc.)

### Testing playback controllers

Testing `change_track` and `translate_uri` is probably the highest priority, since these methods are used to prepare the track and provide its audio URL to Mopidy’s core for playback.

### Testing frontends

Because most frontends will interact with the Mopidy core, it will most likely be necessary to have a full core running for testing purposes:

    self.core = core.Core.start(
                config, backends=[get_backend(config)]).proxy()

It may be advisable to take a quick look at the Pykka API at this point to make sure that you are familiar with `ThreadingActor`, `ThreadingFuture`, and the `proxies` that allow you to access the attributes and methods of the actor directly.

You’ll also need a list of `Track` and a list of URIs in order to populate the core with some simple tracks that can be used for testing:

    class BaseTest(unittest.TestCase):
        tracks = [
            models.Track(uri='my_scheme:track:id1', length=40000),  # Regular track
            models.Track(uri='my_scheme:track:id2', length=None),   # No duration
        ]

    uris = [ 'my_scheme:track:id1', 'my_scheme:track:id2']

In the `setup()` method of your test class, you will then probably need to monkey patch looking up tracks in the library (so that it will always use the lists that you defined), and then populate the core’s tracklist:

    def lookup(uris):
        result = {uri: [] for uri in uris}
        for track in self.tracks:
            if track.uri in result:
                result[track.uri].append(track)
        return result

    self.core.library.lookup = lookup
    self.tl_tracks = self.core.tracklist.add(uris=self.uris).get()

With all of that done you should finally be ready to instantiate your frontend:

    self.frontend = frontend.MyFrontend.start(config(), self.core).proxy()

Keep in mind that the normal core and frontend methods will usually return `pykka.ThreadingFuture` objects, so you will need to add `.get()` at the end of most method calls in order to get to the actual return values.

### Triggering events

There may be test case scenarios that require simulating certain event triggers that your extension’s actors can listen for and respond on. An example for patching the listener to store these events, and then play them back for your actor, may look something like this:

    self.events = []
    self.patcher = mock.patch('mopidy.listener.send')
    self.send_mock = self.patcher.start()

    def send(cls, event, **kwargs):
        self.events.append((event, kwargs))

    self.send_mock.side_effect = send

Once all of the events have been captured, a method like `replay_events()` can be called at the relevant points in the code to have the events fire:

    def replay_events(self, my_actor, until=None):
        while self.events:
            if self.events[0][0] == until:
                break
            event, kwargs = self.events.pop(0)
            frontend.on_event(event, **kwargs).get()

For further details and examples, refer to the /tests directory on the Mopidy development branch.