Add a nicer warning message when you pass the wrong thing to ray.wait() (#1239)

* add warnings

* fix python mode

* Small changes and add tests.

* Fix test failure.

python/ray/tune/trial_runner.py

@@ -156,7 +156,7 @@ class TrialRunner(object):
                 # have been lost
 
     def _process_events(self):
-        [result_id], _ = ray.wait(self._running.keys())
+        [result_id], _ = ray.wait(list(self._running.keys()))
         trial = self._running[result_id]
         del self._running[result_id]
         try:

python/ray/worker.py

@@ -2223,6 +2223,22 @@ def wait(object_ids, num_returns=1, timeout=None, worker=global_worker):
         A list of object IDs that are ready and a list of the remaining object
             IDs.
     """
+
+    if isinstance(object_ids, ray.local_scheduler.ObjectID):
+        raise TypeError(
+            "wait() expected a list of ObjectID, got a single ObjectID")
+
+    if not isinstance(object_ids, list):
+        raise TypeError("wait() expected a list of ObjectID, got {}".format(
+            type(object_ids)))
+
+    if worker.mode != PYTHON_MODE:
+        for object_id in object_ids:
+            if not isinstance(object_id, ray.local_scheduler.ObjectID):
+                raise TypeError(
+                    "wait() expected a list of ObjectID, "
+                    "got list containing {}".format(type(object_id)))
+
     check_connected(worker)
     with log_span("ray:wait", worker=worker):
         check_main_thread()