diff --git a/python/ray/rllib/models/preprocessors.py b/python/ray/rllib/models/preprocessors.py
index e68ffbefc..dd133f2aa 100644
--- a/python/ray/rllib/models/preprocessors.py
+++ b/python/ray/rllib/models/preprocessors.py
@@ -30,6 +30,7 @@ class Preprocessor(object):
         self._obs_space = obs_space
         self._options = options or {}
         self.shape = self._init_shape(obs_space, options)
+        self._size = int(np.product(self.shape))
 
     @PublicAPI
     def _init_shape(self, obs_space, options):
@@ -41,10 +42,14 @@ class Preprocessor(object):
         """Returns the preprocessed observation."""
         raise NotImplementedError
 
+    def write(self, observation, array, offset):
+        """Alternative to transform for more efficient flattening."""
+        array[offset:offset + self._size] = self.transform(observation)
+
     @property
     @PublicAPI
     def size(self):
-        return int(np.product(self.shape))
+        return self._size
 
     @property
     @PublicAPI
@@ -123,6 +128,10 @@ class OneHotPreprocessor(Preprocessor):
         arr[observation] = 1
         return arr
 
+    @override(Preprocessor)
+    def write(self, observation, array, offset):
+        array[offset + observation] = 1
+
 
 class NoPreprocessor(Preprocessor):
     @override(Preprocessor)
@@ -133,6 +142,11 @@ class NoPreprocessor(Preprocessor):
     def transform(self, observation):
         return observation
 
+    @override(Preprocessor)
+    def write(self, observation, array, offset):
+        array[offset:offset + self._size] = np.array(
+            observation, copy=False).ravel()
+
 
 class TupleFlatteningPreprocessor(Preprocessor):
     """Preprocesses each tuple element, then flattens it all into a vector.
@@ -155,11 +169,16 @@ class TupleFlatteningPreprocessor(Preprocessor):
 
     @override(Preprocessor)
     def transform(self, observation):
+        array = np.zeros(self.shape)
+        self.write(observation, array, 0)
+        return array
+
+    @override(Preprocessor)
+    def write(self, observation, array, offset):
         assert len(observation) == len(self.preprocessors), observation
-        return np.concatenate([
-            np.reshape(p.transform(o), [p.size])
-            for (o, p) in zip(observation, self.preprocessors)
-        ])
+        for o, p in zip(observation, self.preprocessors):
+            p.write(o, array, offset)
+            offset += p.size
 
 
 class DictFlatteningPreprocessor(Preprocessor):
@@ -182,14 +201,19 @@ class DictFlatteningPreprocessor(Preprocessor):
 
     @override(Preprocessor)
     def transform(self, observation):
+        array = np.zeros(self.shape)
+        self.write(observation, array, 0)
+        return array
+
+    @override(Preprocessor)
+    def write(self, observation, array, offset):
         if not isinstance(observation, OrderedDict):
             observation = OrderedDict(sorted(list(observation.items())))
         assert len(observation) == len(self.preprocessors), \
             (len(observation), len(self.preprocessors))
-        return np.concatenate([
-            np.reshape(p.transform(o), [p.size])
-            for (o, p) in zip(observation.values(), self.preprocessors)
-        ])
+        for o, p in zip(observation.values(), self.preprocessors):
+            p.write(o, array, offset)
+            offset += p.size
 
 
 @PublicAPI
diff --git a/python/ray/rllib/tests/test_catalog.py b/python/ray/rllib/tests/test_catalog.py
index 9346e1064..fc9b71d2c 100644
--- a/python/ray/rllib/tests/test_catalog.py
+++ b/python/ray/rllib/tests/test_catalog.py
@@ -16,12 +16,12 @@ from ray.rllib.models.visionnet import VisionNetwork
 
 class CustomPreprocessor(Preprocessor):
     def _init_shape(self, obs_space, options):
-        return None
+        return [1]
 
 
 class CustomPreprocessor2(Preprocessor):
     def _init_shape(self, obs_space, options):
-        return None
+        return [1]
 
 
 class CustomModel(Model):