The importer component is responsible for the user-centric workflow that adds music to a library. This is one of the first aspects that a user experiences when using beets: it finds music in the filesystem, groups it into albums, finds corresponding metadata in MusicBrainz, asks the user for intervention, applies changes, and moves/copies files. A description of its user interface is given in /guides/tagger.
The workflow is implemented in the beets.importer module and is distinct from the core logic for matching MusicBrainz metadata (in the beets.autotag module). The workflow is also decoupled from the command-line interface with the hope that, eventually, other (graphical) interfaces can be bolted onto the same importer implementation.
The importer is multithreaded and follows the pipeline pattern. Each pipeline stage is a Python coroutine. The beets.util.pipeline module houses a generic, reusable implementation of a multithreaded pipeline.