Persistence of custom plugin state?

[gryphonmyers]

I can't find any documentation or example plugins that do this, so if the answer is obvious, I apologize in advance. I'm thinking about writing a plugin that needs to persist some custom state (musicbrainz ids that are determined based on a combination of API calls and beets library queries). It does not make sense to extend the beets db models for this purpose - it is plugin-specific state that does not pertain to existing library items or albums. What is considered best practice for storage and retrieval of this kind of data?

[sampsyo]

We actually do not have a general-purpose way to do this! It's common to attach attributes to items or albums for plugin-specific reasons, but we don't have a way that's unattached to the models.

You might consider taking inspiration from the way the importer saves its progress state, which is just storing a pickled data structure in the configuration directory:

beets/beets/importer.py

Lines 69 to 91 in 0c3f428

	# Utilities.
	def _open_state():
	"""Reads the state file, returning a dictionary."""
	try:
	with open(config['statefile'].as_filename(), 'rb') as f:
	return pickle.load(f)
	except Exception as exc:
	# The `pickle` module can emit all sorts of exceptions during
	# unpickling, including ImportError. We use a catch-all
	# exception to avoid enumerating them all (the docs don't even have a
	# full list!).
	log.debug('state file could not be read: {0}', exc)
	return {}
	def _save_state(state):
	"""Writes the state dictionary out to disk."""
	try:
	with open(config['statefile'].as_filename(), 'wb') as f:
	pickle.dump(state, f)
	except OSError as exc:
	log.error('state file could not be written: {0}', exc)