core: Make lookup(uris=...) return dict with all uris

All uris given to lookup should be in the result even if there is no
backend to handle the uri, and the lookup result thus is an empty list.

As a side effect, future.get() is now called in the order of the URIs
in the `uris` list, making it easier to mock out
backend.library.lookup() in core layer tests.

mopidy/core/library.py

@@ -188,20 +188,26 @@ class LibraryController(object):
         if none_set or both_set:
             raise ValueError("One of 'uri' or 'uris' must be set")
 
+        if uri is not None:
+            uris = [uri]
+
         futures = {}
         result = {}
-        backends = self._get_backends_to_uris([uri] if uri else uris)
+        backends = self._get_backends_to_uris(uris)
 
         # TODO: lookup(uris) to backend APIs
         for backend, backend_uris in backends.items():
             for u in backend_uris or []:
                 futures[u] = backend.library.lookup(u)
 
-        for u, future in futures.items():
-            result[u] = future.get()
+        for u in uris:
+            if u in futures:
+                result[u] = futures[u].get()
+            else:
+                result[u] = []
 
         if uri:
-            return result.get(uri, [])
+            return result[uri]
         return result
 
     def refresh(self, uri=None):

tests/core/test_library.py

@@ -168,13 +168,20 @@ class CoreLibraryTest(unittest.TestCase):
         result = self.core.library.lookup(uris=['dummy1:a', 'dummy2:a'])
         self.assertEqual(result, {'dummy2:a': [5678], 'dummy1:a': [1234]})
 
-    def test_lookup_returns_nothing_for_dummy3_track(self):
+    def test_lookup_uri_returns_empty_list_for_dummy3_track(self):
         result = self.core.library.lookup('dummy3:a')
 
         self.assertEqual(result, [])
         self.assertFalse(self.library1.lookup.called)
         self.assertFalse(self.library2.lookup.called)
 
+    def test_lookup_uris_returns_empty_list_for_dummy3_track(self):
+        result = self.core.library.lookup(uris=['dummy3:a'])
+
+        self.assertEqual(result, {'dummy3:a': []})
+        self.assertFalse(self.library1.lookup.called)
+        self.assertFalse(self.library2.lookup.called)
+
     def test_refresh_with_uri_selects_dummy1_backend(self):
         self.core.library.refresh('dummy1:a')