only load plugins from specified modules

Eliminate the __subclasses__ trick for finding all plugins. Now we explicitly
look in each plugin module for a plugin class. This allows us to import plugin
modules with unintentionally loading them. This lets us reuse the image
embedding machinery without copypasta.

beets/plugins.py

@@ -22,7 +22,6 @@ from collections import defaultdict
 from beets import mediafile
 
 PLUGIN_NAMESPACE = 'beetsplug'
-DEFAULT_PLUGINS = []
 
 # Plugins using the Last.fm API can share the same API key.
 LASTFM_KEY = '2dc3914abf35f0d9c92d97d8f8e42b43'
@@ -151,24 +150,29 @@ class BeetsPlugin(object):
             return func
         return helper
 
+_classes = set()
 def load_plugins(names=()):
     """Imports the modules for a sequence of plugin names. Each name
     must be the name of a Python module under the "beetsplug" namespace
     package in sys.path; the module indicated should contain the
-    BeetsPlugin subclasses desired. A default set of plugins is also
-    loaded.
+    BeetsPlugin subclasses desired.
     """
-    for name in itertools.chain(names, DEFAULT_PLUGINS):
+    for name in names:
         modname = '%s.%s' % (PLUGIN_NAMESPACE, name)
         try:
             try:
-                __import__(modname, None, None)
+                namespace = __import__(modname, None, None)
             except ImportError as exc:
                 # Again, this is hacky:
                 if exc.args[0].endswith(' ' + name):
                     log.warn('** plugin %s not found' % name)
                 else:
                     raise
+            else:
+                for obj in getattr(namespace, name).__dict__.values():
+                    if isinstance(obj, type) and issubclass(obj, BeetsPlugin):
+                        _classes.add(obj)
+
         except:
             log.warn('** error loading plugin %s' % name)
             log.warn(traceback.format_exc())
@@ -181,7 +185,7 @@ def find_plugins():
     """
     load_plugins()
     plugins = []
-    for cls in BeetsPlugin.__subclasses__():
+    for cls in _classes:
         # Only instantiate each plugin class once.
         if cls not in _instances:
             _instances[cls] = cls()

beetsplug/convert.py

@@ -22,34 +22,14 @@ from subprocess import Popen, PIPE
 import imghdr
 
 from beets.plugins import BeetsPlugin
-from beets import ui, library, util, mediafile
+from beets import ui, library, util
+from beetsplug.embedart import _embed
 
 log = logging.getLogger('beets')
 DEVNULL = open(os.devnull, 'wb')
 conf = {}
 
 
-def _embed(path, items):
-    """Embed an image file, located at `path`, into each item.
-    """
-    data = open(util.syspath(path), 'rb').read()
-    kindstr = imghdr.what(None, data)
-    if kindstr not in ('jpeg', 'png'):
-        log.error('A file of type %s is not allowed as coverart.' % kindstr)
-        return
-    log.debug('Embedding album art.')
-    for item in items:
-        try:
-            f = mediafile.MediaFile(util.syspath(item.path))
-        except mediafile.UnreadableFileError as exc:
-            log.warn('Could not embed art in {0}: {1}'.format(
-                repr(item.path), exc
-            ))
-            continue
-        f.art = data
-        f.save()
-
-
 def encode(source, dest):
     log.info('Started encoding ' + source)
     temp_dest = dest + '~'

docs/changelog.rst

@@ -48,6 +48,8 @@ Changelog
 * Fix a VFS bug leading to a crash in the :doc:`/plugins/bpd` when files had
   non-ASCII extensions.
 * Add a human-readable error message when writing files' tags fails.
+* Changed plugin loading so that modules can be imported without
+  unintentionally loading the plugins they contain.
 
 .. _Tomahawk resolver: http://beets.radbox.org/blog/tomahawk-resolver.html