Plugins can define types of flexible fields

This partially solves #647.

beets/plugins.py

@@ -31,6 +31,14 @@ LASTFM_KEY = '2dc3914abf35f0d9c92d97d8f8e42b43'
 log = logging.getLogger('beets')
 
 
+class PluginConflictException(Exception):
+    """Indicates that the services provided by one plugin conflict with
+    those of another.
+
+    For example two plugins may define different types for flexible fields.
+    """
+
+
 # Managing the plugins themselves.
 
 class BeetsPlugin(object):
@@ -247,6 +255,22 @@ def queries():
     return out
 
 
+def types(model_cls):
+    # Gives us `item_types` and `album_types`
+    attr_name = '{0}_types'.format(model_cls.__name__.lower())
+    types = {}
+    for plugin in find_plugins():
+        plugin_types = getattr(plugin, attr_name, {})
+        for field in plugin_types:
+            if field in types:
+                raise PluginConflictException(
+                    u'Plugin {0} defines flexible field {1} '
+                    'which has already been defined.'
+                    .format(plugin.name,))
+        types.update(plugin_types)
+    return types
+
+
 def track_distance(item, info):
     """Gets the track distance calculated by all loaded plugins.
     Returns a Distance object.

beets/ui/__init__.py

@@ -869,6 +869,8 @@ def _setup(options, lib=None):
     if lib is None:
         lib = _open_library(config)
         plugins.send("library_opened", lib=lib)
+    library.Item._types = plugins.types(library.Item)
+    library.Album._types = plugins.types(library.Album)
 
     return subcommands, plugins, lib