Give better error for old-style actor classes. (#3793)

python/ray/actor.py

@@ -779,6 +779,13 @@ class ActorHandle(object):
 
 def make_actor(cls, num_cpus, num_gpus, resources, actor_method_cpus,
                checkpoint_interval, max_reconstructions):
+    # Give an error if cls is an old-style class.
+    if not issubclass(cls, object):
+        raise TypeError(
+            "The @ray.remote decorator cannot be applied to old-style "
+            "classes. In Python 2, you must declare the class with "
+            "'class ClassName(object):' instead of 'class ClassName:'.")
+
     if checkpoint_interval is None:
         checkpoint_interval = -1
     if max_reconstructions is None:

test/actor_test.py

@@ -98,6 +98,16 @@ def test_actor_init_error_propagated(ray_start_regular):
         ray.get(actor.foo.remote())
 
 
+@pytest.mark.skipif(
+    sys.version_info >= (3, 0), reason="This test requires Python 2.")
+def test_old_style_error(ray_start_regular):
+    with pytest.raises(TypeError):
+
+        @ray.remote
+        class Actor:
+            pass
+
+
 def test_keyword_args(ray_start_regular):
     @ray.remote
     class Actor(object):