Writing Plugins

A beets plugin is just a Python module inside the beetsplug namespace package. (Check out this Stack Overflow question about namespace packages if you haven’t heard of them.) So, to make one, create a directory called beetsplug and put two files in it: one called __init__.py and one called myawesomeplugin.py (but don’t actually call it that). Your directory structure should look like this:


Then, you’ll need to put this stuff in __init__.py to make beetsplug a namespace package:

from pkgutil import extend_path
__path__ = extend_path(__path__, __name__)

That’s all for __init__.py; you can can leave it alone. The meat of your plugin goes in myawesomeplugin.py. There, you’ll have to import the beets.plugins module and define a subclass of the BeetsPlugin class found therein. Here’s a skeleton of a plugin file:

from beets.plugins import BeetsPlugin

class MyPlugin(BeetsPlugin):

Once you have your BeetsPlugin subclass, there’s a variety of things your plugin can do. (Read on!)

To use your new plugin, make sure your beetsplug directory is in the Python path (using PYTHONPATH or by installing in a virtualenv, for example). Then, as described above, edit your config.yaml to include plugins: myawesomeplugin (substituting the name of the Python module containing your plugin).

Add Commands to the CLI

Plugins can add new subcommands to the beet command-line interface. Define the plugin class’ commands() method to return a list of Subcommand objects. (The Subcommand class is defined in the beets.ui module.) Here’s an example plugin that adds a simple command:

from beets.plugins import BeetsPlugin
from beets.ui import Subcommand

my_super_command = Subcommand('super', help='do something super')
def say_hi(lib, opts, args):
    print "Hello everybody! I'm a plugin!"
my_super_command.func = say_hi

class SuperPlug(BeetsPlugin):
    def commands(self):
        return [my_super_command]

To make a subcommand, invoke the constructor like so: Subcommand(name, parser, help, aliases). The name parameter is the only required one and should just be the name of your command. parser can be an OptionParser instance, but it defaults to an empty parser (you can extend it later). help is a description of your command, and aliases is a list of shorthand versions of your command name.

You’ll need to add a function to your command by saying mycommand.func = myfunction. This function should take the following parameters: lib (a beets Library object) and opts and args (command-line options and arguments as returned by OptionParser.parse_args).

The function should use any of the utility functions defined in beets.ui. Try running pydoc beets.ui to see what’s available.

You can add command-line options to your new command using the parser member of the Subcommand class, which is a CommonOptionsParser instance. Just use it like you would a normal OptionParser in an independent script. Note that it offers several methods to add common options: --album, --path and --format. This feature is versatile and extensively documented, try pydoc beets.ui.CommonOptionsParser for more information.

Listen for Events

Event handlers allow plugins to run code whenever something happens in beets’ operation. For instance, a plugin could write a log message every time an album is successfully autotagged or update MPD’s index whenever the database is changed.

You can “listen” for events using the BeetsPlugin.listen decorator. Here’s an example:

from beets.plugins import BeetsPlugin

class SomePlugin(BeetsPlugin):

def loaded():
    print 'Plugin loaded!'

Pass the name of the event in question to the listen decorator.

Note that if you want to access an attribute of your plugin (e.g. config or log) you’ll have to define a method and not a function. Here is the usual registration process in this case:

from beets.plugins import BeetsPlugin

class SomePlugin(BeetsPlugin):
  def __init__(self):
    super(SomePlugin, self).__init__()
    self.register_listener('pluginload', self.loaded)

  def loaded(self):
    self._log.info('Plugin loaded!')

The events currently available are:

  • pluginload: called after all the plugins have been loaded after the beet command starts
  • import: called after a beet import command finishes (the lib keyword argument is a Library object; paths is a list of paths (strings) that were imported)
  • album_imported: called with an Album object every time the import command finishes adding an album to the library. Parameters: lib, album
  • item_copied: called with an Item object whenever its file is copied. Parameters: item, source path, destination path
  • item_imported: called with an Item object every time the importer adds a singleton to the library (not called for full-album imports). Parameters: lib, item
  • before_item_moved: called with an Item object immediately before its file is moved. Parameters: item, source path, destination path
  • item_moved: called with an Item object whenever its file is moved. Parameters: item, source path, destination path
  • item_linked: called with an Item object whenever a symlink is created for a file. Parameters: item, source path, destination path
  • item_removed: called with an Item object every time an item (singleton or album’s part) is removed from the library (even when its file is not deleted from disk).
  • write: called with an Item object, a path, and a tags dictionary just before a file’s metadata is written to disk (i.e., just before the file on disk is opened). Event handlers may change the tags dictionary to customize the tags that are written to the media file. Event handlers may also raise a library.FileOperationError exception to abort the write operation. Beets will catch that exception, print an error message and continue.
  • after_write: called with an Item object after a file’s metadata is written to disk (i.e., just after the file on disk is closed).
  • import_task_created: called immediately after an import task is initialized. Plugins can use this to, for example, change imported files of a task before anything else happens. It’s also possible to replace the task with another task by returning a list of tasks. This list can contain zero or more ImportTask`s. Returning an empty list will stop the task. Parameters: ``task` (an ImportTask) and session (an ImportSession).
  • import_task_start: called when before an import task begins processing. Parameters: task and session.
  • import_task_apply: called after metadata changes have been applied in an import task. This is called on the same thread as the UI, so use this sparingly and only for tasks that can be done quickly. For most plugins, an import pipeline stage is a better choice (see Add Import Pipeline Stages). Parameters: task and session.
  • import_task_choice: called after a decision has been made about an import task. This event can be used to initiate further interaction with the user. Use task.choice_flag to determine or change the action to be taken. Parameters: task and session.
  • import_task_files: called after an import task finishes manipulating the filesystem (copying and moving files, writing metadata tags). Parameters: task and session.
  • library_opened: called after beets starts up and initializes the main Library object. Parameter: lib.
  • database_change: a modification has been made to the library database. The change might not be committed yet. Parameters: lib and model.
  • cli_exit: called just before the beet command-line program exits. Parameter: lib.
  • import_begin: called just before a beet import session starts up. Parameter: session.
  • trackinfo_received: called after metadata for a track item has been fetched from a data source, such as MusicBrainz. You can modify the tags that the rest of the pipeline sees on a beet import operation or during later adjustments, such as mbsync. Slow handlers of the event can impact the operation, since the event is fired for any fetched possible match before the user (or the autotagger machinery) gets to see the match. Parameter: info.
  • albuminfo_received: like trackinfo_received, the event indicates new metadata for album items. The parameter is an AlbumInfo object instead of a TrackInfo. Parameter: info.
  • before_choose_candidate: called before the user is prompted for a decision during a beet import interactive session. Plugins can use this event for appending choices to the prompt by returning a list of PromptChoices. Parameters: task and session.

The included mpdupdate plugin provides an example use case for event listeners.

Extend the Autotagger

Plugins can also enhance the functionality of the autotagger. For a comprehensive example, try looking at the chroma plugin, which is included with beets.

A plugin can extend three parts of the autotagger’s process: the track distance function, the album distance function, and the initial MusicBrainz search. The distance functions determine how “good” a match is at the track and album levels; the initial search controls which candidates are presented to the matching algorithm. Plugins implement these extensions by implementing four methods on the plugin class:

  • track_distance(self, item, info): adds a component to the distance function (i.e., the similarity metric) for individual tracks. item is the track to be matched (an Item object) and info is the TrackInfo object that is proposed as a match. Should return a (dist, dist_max) pair of floats indicating the distance.
  • album_distance(self, items, album_info, mapping): like the above, but compares a list of items (representing an album) to an album-level MusicBrainz entry. items is a list of Item objects; album_info is an AlbumInfo object; and mapping is a dictionary that maps Items to their corresponding TrackInfo objects.
  • candidates(self, items, artist, album, va_likely): given a list of items comprised by an album to be matched, return a list of AlbumInfo objects for candidate albums to be compared and matched.
  • item_candidates(self, item, artist, album): given a singleton item, return a list of TrackInfo objects for candidate tracks to be compared and matched.
  • album_for_id(self, album_id): given an ID from user input or an album’s tags, return a candidate AlbumInfo object (or None).
  • track_for_id(self, track_id): given an ID from user input or a file’s tags, return a candidate TrackInfo object (or None).

When implementing these functions, you may want to use the functions from the beets.autotag and beets.autotag.mb modules, both of which have somewhat helpful docstrings.

Read Configuration Options

Plugins can configure themselves using the config.yaml file. You can read configuration values in two ways. The first is to use self.config within your plugin class. This gives you a view onto the configuration values in a section with the same name as your plugin’s module. For example, if your plugin is in greatplugin.py, then self.config will refer to options under the greatplugin: section of the config file.

For example, if you have a configuration value called “foo”, then users can put this in their config.yaml:

    foo: bar

To access this value, say self.config['foo'].get() at any point in your plugin’s code. The self.config object is a view as defined by the Confit library.

If you want to access configuration values outside of your plugin’s section, import the config object from the beets module. That is, just put from beets import config at the top of your plugin and access values from there.

If your plugin provides configuration values for sensitive data (e.g., passwords, API keys, ...), you should add these to the config so they can be redacted automatically when users dump their config. This can be done by setting each value’s redact flag, like so:

self.config['password'].redact = True

Add Path Format Functions and Fields

Beets supports function calls in its path format syntax (see Path Formats). Beets includes a few built-in functions, but plugins can register new functions by adding them to the template_funcs dictionary.

Here’s an example:

class MyPlugin(BeetsPlugin):
    def __init__(self):
        super(MyPlugin, self).__init__()
        self.template_funcs['initial'] = _tmpl_initial

def _tmpl_initial(text):
    if text:
        return text[0].upper()
        return u''

This plugin provides a function %initial to path templates where %initial{$artist} expands to the artist’s initial (its capitalized first character).

Plugins can also add template fields, which are computed values referenced as $name in templates. To add a new field, add a function that takes an Item object to the template_fields dictionary on the plugin object. Here’s an example that adds a $disc_and_track field:

class MyPlugin(BeetsPlugin):
    def __init__(self):
        super(MyPlugin, self).__init__()
        self.template_fields['disc_and_track'] = _tmpl_disc_and_track

def _tmpl_disc_and_track(item):
    """Expand to the disc number and track number if this is a
    multi-disc release. Otherwise, just expands to the track
    if item.disctotal > 1:
        return u'%02i.%02i' % (item.disc, item.track)
        return u'%02i' % (item.track)

With this plugin enabled, templates can reference $disc_and_track as they can any standard metadata field.

This field works for item templates. Similarly, you can register album template fields by adding a function accepting an Album argument to the album_template_fields dict.

Extend MediaFile

MediaFile is the file tag abstraction layer that beets uses to make cross-format metadata manipulation simple. Plugins can add fields to MediaFile to extend the kinds of metadata that they can easily manage.

The MediaFile class uses MediaField descriptors to provide access to file tags. Have a look at the beets.mediafile source code to learn how to use this descriptor class. If you have created a descriptor you can add it through your plugins add_media_field() method.

BeetsPlugin.add_media_field(name, descriptor)

Add a field that is synchronized between media files and items.

When a media field is added item.write() will set the name property of the item’s MediaFile to item[name] and save the changes. Similarly item.read() will set item[name] to the value of the name property of the media file.

descriptor must be an instance of mediafile.MediaField.

Here’s an example plugin that provides a meaningless new field “foo”:

class FooPlugin(BeetsPlugin):
    def __init__(self):
        field = mediafile.MediaField(
        self.add_media_field('foo', field)

item = Item.from_path('/path/to/foo/tag.mp3')
assert item['foo'] == 'spam'

item['foo'] == 'ham'
# The "foo" tag of the file is now "ham"

Add Import Pipeline Stages

Many plugins need to add high-latency operations to the import workflow. For example, a plugin that fetches lyrics from the Web would, ideally, not block the progress of the rest of the importer. Beets allows plugins to add stages to the parallel import pipeline.

Each stage is run in its own thread. Plugin stages run after metadata changes have been applied to a unit of music (album or track) and before file manipulation has occurred (copying and moving files, writing tags to disk). Multiple stages run in parallel but each stage processes only one task at a time and each task is processed by only one stage at a time.

Plugins provide stages as functions that take two arguments: config and task, which are ImportSession and ImportTask objects (both defined in beets.importer). Add such a function to the plugin’s import_stages field to register it:

from beets.plugins import BeetsPlugin
class ExamplePlugin(BeetsPlugin):
    def __init__(self):
        super(ExamplePlugin, self).__init__()
        self.import_stages = [self.stage]
    def stage(self, session, task):
        print('Importing something!')

Extend the Query Syntax

You can add new kinds of queries to beets’ query syntax indicated by a prefix. As an example, beets already supports regular expression queries, which are indicated by a colon prefix—plugins can do the same.

To do so, define a subclass of the Query type from the beets.dbcore.query module. Then, in the queries method of your plugin class, return a dictionary mapping prefix strings to query classes.

One simple kind of query you can extend is the FieldQuery, which implements string comparisons on fields. To use it, create a subclass inheriting from that class and override the value_match class method. (Remember the @classmethod decorator!) The following example plugin declares a query using the @ prefix to delimit exact string matches. The plugin will be used if we issue a command like beet ls @something or beet ls artist:@something:

from beets.plugins import BeetsPlugin
from beets.dbcore import FieldQuery

class ExactMatchQuery(FieldQuery):
    def value_match(self, pattern, val):
        return pattern == val

class ExactMatchPlugin(BeetsPlugin):
    def queries(self):
        return {
            '@': ExactMatchQuery

Flexible Field Types

If your plugin uses flexible fields to store numbers or other non-string values, you can specify the types of those fields. A rating plugin, for example, might want to declare that the rating field should have an integer type:

from beets.plugins import BeetsPlugin
from beets.dbcore import types

class RatingPlugin(BeetsPlugin):
    item_types = {'rating': types.INTEGER}

    def album_types(self):
        return {'rating': types.INTEGER}

A plugin may define two attributes: item_types and album_types. Each of those attributes is a dictionary mapping a flexible field name to a type instance. You can find the built-in types in the beets.dbcore.types and beets.library modules or implement your own type by inheriting from the Type class.

Specifying types has several advantages:

  • Code that accesses the field like item['my_field'] gets the right type (instead of just a string).
  • You can use advanced queries (like ranges) from the command line.
  • User input for flexible fields may be validated and converted.


Each plugin object has a _log attribute, which is a Logger from the standard Python logging module. The logger is set up to PEP 3101, str.format-style string formatting. So you can write logging calls like this:

self._log.debug(u'Processing {0.title} by {0.artist}', item)

When beets is in verbose mode, plugin messages are prefixed with the plugin name to make them easier to see.

Which messages will be logged depends on the logging level and the action performed:

  • Inside import stages and event handlers, the default is WARNING messages and above.
  • Everywhere else, the default is INFO or above.

The verbosity can be increased with --verbose (-v) flags: each flags lowers the level by a notch. That means that, with a single -v flag, event handlers won’t have their DEBUG messages displayed, but command functions (for example) will. With -vv on the command line, DEBUG messages will be displayed everywhere.

This addresses a common pattern where plugins need to use the same code for a command and an import stage, but the command needs to print more messages than the import stage. (For example, you’ll want to log “found lyrics for this song” when you’re run explicitly as a command, but you don’t want to noisily interrupt the importer interface when running automatically.)

Append Prompt Choices

Plugins can also append choices to the prompt presented to the user during an import session.

To do so, add a listener for the before_choose_candidate event, and return a list of PromptChoices that represent the additional choices that your plugin shall expose to the user:

from beets.plugins import BeetsPlugin
from beets.ui.commands import PromptChoice

class ExamplePlugin(BeetsPlugin):
    def __init__(self):
        super(ExamplePlugin, self).__init__()

    def before_choose_candidate_event(self, session, task):
        return [PromptChoice('p', 'Print foo', self.foo),
                PromptChoice('d', 'Do bar', self.bar)]

    def foo(self, session, task):
        print('User has chosen "Print foo"!')

    def bar(self, session, task):
        print('User has chosen "Do bar"!')

The previous example modifies the standard prompt:

# selection (default 1), Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort?

by appending two additional options (Print foo and Do bar):

# selection (default 1), Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort, Print foo, Do bar?

If the user selects a choice, the callback attribute of the corresponding PromptChoice will be called. It is the responsibility of the plugin to check for the status of the import session and decide the choices to be appended: for example, if a particular choice should only be presented if the album has no candidates, the relevant checks against task.candidates should be performed inside the plugin’s before_choose_candidate_event accordingly.

Please make sure that the short letter for each of the choices provided by the plugin is not already in use: the importer will emit a warning and discard all but one of the choices using the same letter, giving priority to the core importer prompt choices. As a reference, the following characters are used by the choices on the core importer prompt, and hence should not be used: a, s, u, t, g, e, i, b.

Additionally, the callback function can optionally specify the next action to be performed by returning a importer.action value. It may also return a autotag.Proposal value to update the set of current proposals to be considered.