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, including:

  • 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, introducing:

  • 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.

Extension project template

We’ve made a Copier project template for creating new Mopidy extensions.

By running a single command, you’re asked a few questions about the name of your extension, and then your answers are used to create a folder structure similar to the above, with all the needed files and most of the details filled in for you.

See the readme of mopidy-ext-template for further details.

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 registered on PyPI. The name of the distribution on PyPI would be something like “mopidy-soundspot”. 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
    pyproject.toml          # Project metadata and build configuration
    LICENSE                 # The license text
    README.md               # Document what it is and how to use it
    src/
        mopidy_soundspot/   # Your code
            __init__.py
            ext.conf        # Default config for the extension
            ...
    tests/
        __init__.py
        test_extension.py   # Tests for the extension

See the above project template for examples of each of the files.

Extension registration

For Mopidy to find your extension, the extension must register an entry point of the mopidy.ext type.

This is done by adding a section like this to your pyproject.toml file:

[project.entry-points."mopidy.ext"]
soundspot = "mopidy_soundspot:Extension"

In this example, the first part, soundspot, is your extensions “short name”, which is used to identify your extension in the Mopidy configuration file. The second part, mopidy_soundspot:Extension, is the fully qualified name of your Python module and extension class.

Extension class

The root of your Python package 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 project.entry-points part of pyproject.toml.

Any imports of other files in your extension, outside of Mopidy and its 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 `project.entry-points part of pyproject.toml, 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 src/mopidy_soundspot/__init__.py:

import logging
import pathlib
from importlib.metadata import version

from mopidy import config, exceptions, ext

__version__ = version('mopidy-soundspot')

# 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):
        return config.read(pathlib.Path(__file__).parent / "ext.conf")

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

    def get_command(self):
        # To extend the `mopidy` command line interface:
        from .commands import SoundspotCommand
        return SoundspotCommand()

    def validate_environment(self):
        # Any manual checks of the environment to fail early.
        # Dependencies described by pyproject.toml 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 src/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().__init__()
        self.config = config
        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().__init__()
        self.config = config
        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().__init__()
        self.add_argument("--foo")

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

Example web application

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

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(__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/4.0.0 Python/3.13.2'

Examples

Most Mopidy extensions that make HTTP requests use either the httpx or Requests library to do so.

Example using HTTPX

If you’re using HTTPX, you can create a session object like this:

import httpx from mopidy import httpclient

import mopidy_soundspot

client = httpx.Client(

proxy=httpclient.format_proxy(proxy_config), headers={

“user-agent”: httpclient.format_user_agent(

f”{mopidy_soundspot.Extension.dist_name}/{mopidy_soundspot.__version__}”

),

}

) response = client.get(”https://example.com”)

For further details, see HTTPX’ docs on clients.

Example using Requests

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:

import requests
from mopidy import httpclient

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")

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 third 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 backend and/or frontend Pykka 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:

  • pytest-httpx - A pytest plugin for mocking HTTPX requests.

  • 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 mock out the API’s that you use to avoid any unintended requests from being made by your backend during testing.

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:

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 in the Mopidy repo.