Fail nicely when optional dependencies are missing

mopidy/__init__.py

@@ -22,6 +22,9 @@ class MopidyException(Exception):
 class SettingsError(MopidyException):
     pass
 
+class OptionalDependencyError(MopidyException):
+    pass
+
 from mopidy import settings as default_settings_module
 from mopidy.utils.settings import SettingsProxy
 settings = SettingsProxy(default_settings_module)

mopidy/core.py

@@ -2,7 +2,7 @@ import logging
 import multiprocessing
 import optparse
 
-from mopidy import get_version, settings
+from mopidy import get_version, settings, OptionalDependencyError
 from mopidy.utils import get_class
 from mopidy.utils.log import setup_logging
 from mopidy.utils.path import get_or_create_folder, get_or_create_file
@@ -69,9 +69,12 @@ class CoreProcess(BaseProcess):
     def setup_frontends(self, core_queue, backend):
         frontends = []
         for frontend_class_name in settings.FRONTENDS:
-            frontend = get_class(frontend_class_name)(core_queue, backend)
-            frontend.start()
-            frontends.append(frontend)
+            try:
+                frontend = get_class(frontend_class_name)(core_queue, backend)
+                frontend.start()
+                frontends.append(frontend)
+            except OptionalDependencyError as e:
+                logger.info(u'Disabled: %s (%s)', frontend_class_name, e)
         return frontends
 
     def process_message(self, message):

mopidy/frontends/lastfm.py

@@ -3,8 +3,11 @@ import multiprocessing
 import socket
 import time
 
-# TODO Log nice error message if pylast isn't found
-import pylast
+try:
+    import pylast
+except ImportError as e:
+    from mopidy import OptionalDependencyError
+    raise OptionalDependencyError(e)
 
 from mopidy import get_version, settings, SettingsError
 from mopidy.frontends.base import BaseFrontend