core: Warn if backend does not implement as_list()

Fixes #1080

mopidy/core/playlists.py

@@ -1,6 +1,6 @@
 from __future__ import absolute_import, unicode_literals
 
-import itertools
+import logging
 import urlparse
 
 import pykka
@@ -10,6 +10,9 @@
 from mopidy.utils.deprecation import deprecated_property
 
 
+logger = logging.getLogger(__name__)
+
+
 class PlaylistsController(object):
     pykka_traversable = True
 
@@ -29,11 +32,20 @@ def as_list(self):
 
         .. versionadded:: 1.0
         """
-        futures = [
-            b.playlists.as_list()
-            for b in self.backends.with_playlists.values()]
-        results = pykka.get_all(futures)
-        return list(itertools.chain(*results))
+        futures = {
+            b.actor_ref.actor_class.__name__: b.playlists.as_list()
+            for b in set(self.backends.with_playlists.values())}
+
+        results = []
+        for backend_name, future in futures.items():
+            try:
+                results.extend(future.get())
+            except NotImplementedError:
+                logger.warning(
+                    '%s does not implement playlists.as_list(). '
+                    'Please upgrade it.', backend_name)
+
+        return results
 
     def get_items(self, uri):
         """

tests/core/test_playlists.py

@@ -31,10 +31,12 @@ def setUp(self):  # noqa: N802
         self.sp2.lookup.return_value.get.side_effect = [self.pl2a, self.pl2b]
 
         self.backend1 = mock.Mock()
+        self.backend1.actor_ref.actor_class.__name__ = 'Backend1'
         self.backend1.uri_schemes.get.return_value = ['dummy1']
         self.backend1.playlists = self.sp1
 
         self.backend2 = mock.Mock()
+        self.backend2.actor_ref.actor_class.__name__ = 'Backend2'
         self.backend2.uri_schemes.get.return_value = ['dummy2']
         self.backend2.playlists = self.sp2
 
@@ -55,6 +57,15 @@ def test_as_list_combines_result_from_backends(self):
         self.assertIn(self.plr2a, result)
         self.assertIn(self.plr2b, result)
 
+    def test_as_list_ignores_backends_that_dont_support_it(self):
+        self.sp2.as_list.return_value.get.side_effect = NotImplementedError
+
+        result = self.core.playlists.as_list()
+
+        self.assertEqual(len(result), 2)
+        self.assertIn(self.plr1a, result)
+        self.assertIn(self.plr1b, result)
+
     def test_get_items_selects_the_matching_backend(self):
         ref = Ref.track()
         self.sp2.get_items.return_value.get.return_value = [ref]