diff --git a/python/ray/rllib/agents/mock.py b/python/ray/rllib/agents/mock.py
index 8cd8c08b4695..0b7d77c2a76f 100644
--- a/python/ray/rllib/agents/mock.py
+++ b/python/ray/rllib/agents/mock.py
@@ -20,6 +20,10 @@ class _MockTrainer(Trainer):
         "num_workers": 0,
     })
 
+    @classmethod
+    def default_resource_request(cls, config):
+        return None
+
     def _init(self, config, env_creator):
         self.info = None
         self.restored = False
diff --git a/python/ray/rllib/examples/custom_train_fn.py b/python/ray/rllib/examples/custom_train_fn.py
index 3ae418ee7252..cc7b55e70d5d 100644
--- a/python/ray/rllib/examples/custom_train_fn.py
+++ b/python/ray/rllib/examples/custom_train_fn.py
@@ -40,13 +40,9 @@ def my_train_fn(config, reporter):
 
 if __name__ == "__main__":
     ray.init()
-    tune.run(
-        my_train_fn,
-        resources_per_trial={
-            "cpu": 1,
-        },
-        config={
-            "lr": 0.01,
-            "num_workers": 0,
-        },
-    )
+    config = {
+        "lr": 0.01,
+        "num_workers": 0,
+    }
+    resources = PPOTrainer.default_resource_request(config).to_json()
+    tune.run(my_train_fn, resources_per_trial=resources, config=config)
diff --git a/python/ray/tune/trainable.py b/python/ray/tune/trainable.py
index c10934896bcc..bb70e2b39434 100644
--- a/python/ray/tune/trainable.py
+++ b/python/ray/tune/trainable.py
@@ -21,7 +21,6 @@
                              TIMESTEPS_THIS_ITER, DONE, TIMESTEPS_TOTAL,
                              EPISODES_THIS_ITER, EPISODES_TOTAL,
                              TRAINING_ITERATION, RESULT_DUPLICATE)
-from ray.tune.trial import Resources
 
 logger = logging.getLogger(__name__)
 
@@ -96,7 +95,7 @@ def default_resource_request(cls, config):
         allocation, so the user does not need to.
         """
 
-        return Resources(cpu=1, gpu=0)
+        return None
 
     @classmethod
     def resource_help(cls, config):
diff --git a/python/ray/tune/trial.py b/python/ray/tune/trial.py
index 91ea941b8cf0..cb9351f9adf8 100644
--- a/python/ray/tune/trial.py
+++ b/python/ray/tune/trial.py
@@ -139,6 +139,9 @@ def subtract(cls, original, to_remove):
         return Resources(cpu, gpu, extra_cpu, extra_gpu, new_custom_res,
                          extra_custom_res)
 
+    def to_json(self):
+        return resources_to_json(self)
+
 
 def json_to_resources(data):
     if data is None or data == "null":
@@ -275,9 +278,20 @@ def __init__(self,
         self.config = config or {}
         self.local_dir = local_dir  # This remains unexpanded for syncing.
         self.experiment_tag = experiment_tag
-        self.resources = (
-            resources
-            or self._get_trainable_cls().default_resource_request(self.config))
+        trainable_cls = self._get_trainable_cls()
+        if trainable_cls and hasattr(trainable_cls,
+                                     "default_resource_request"):
+            default_resources = trainable_cls.default_resource_request(
+                self.config)
+            if default_resources:
+                if resources:
+                    raise ValueError(
+                        "Resources for {} have been automatically set to {} "
+                        "by its `default_resource_request()` method. Please "
+                        "clear the `resources_per_trial` option.".format(
+                            trainable_cls, default_resources))
+                resources = default_resources
+        self.resources = resources or Resources(cpu=1, gpu=0)
         self.stopping_criterion = stopping_criterion or {}
         self.upload_dir = upload_dir
         self.loggers = loggers