Raise warning in PixelObservationWrapper if render_mode is None

gym/wrappers/pixel_observation.py

@@ -7,7 +7,7 @@
 import numpy as np
 
 import gym
-from gym import spaces
+from gym import logger, spaces
 
 STATE_KEY = "state"
 
@@ -24,21 +24,21 @@ class PixelObservationWrapper(gym.ObservationWrapper):
 
     Example:
         >>> import gym
-        >>> env = PixelObservationWrapper(gym.make('CarRacing-v1'))
+        >>> env = PixelObservationWrapper(gym.make('CarRacing-v1', render_mode="single_rgb_array"))
         >>> obs = env.reset()
         >>> obs.keys()
         odict_keys(['pixels'])
         >>> obs['pixels'].shape
         (400, 600, 3)
-        >>> env = PixelObservationWrapper(gym.make('CarRacing-v1'), pixels_only=False)
+        >>> env = PixelObservationWrapper(gym.make('CarRacing-v1', render_mode="single_rgb_array"), pixels_only=False)
         >>> obs = env.reset()
         >>> obs.keys()
         odict_keys(['state', 'pixels'])
         >>> obs['state'].shape
         (96, 96, 3)
         >>> obs['pixels'].shape
         (400, 600, 3)
-        >>> env = PixelObservationWrapper(gym.make('CarRacing-v1'), pixel_keys=('obs',))
+        >>> env = PixelObservationWrapper(gym.make('CarRacing-v1', render_mode="single_rgb_array"), pixel_keys=('obs',))
         >>> obs = env.reset()
         >>> obs.keys()
         odict_keys(['obs'])
@@ -92,8 +92,16 @@ def __init__(
                 f"Found key '{key}' in render_kwargs but not in pixel_keys."
             )
 
+        default_render_kwargs = {}
+        if not env.render_mode:
+            default_render_kwargs = {"mode": "rgb_array"}
+            logger.warn(
+                "env.render_mode must be specified to use PixelObservationWrapper:"
+                "`gym.make(env_name, render_mode='single_rgb_array')`."
+            )
+
         for key in pixel_keys:
-            render_kwargs.setdefault(key, {})
+            render_kwargs.setdefault(key, default_render_kwargs)
 
         wrapped_observation_space = env.observation_space
 
@@ -130,6 +138,12 @@ def __init__(
             pixels = self.env.render(**render_kwargs[pixel_key])
             pixels: np.ndarray = pixels[-1] if isinstance(pixels, List) else pixels
 
+            if not hasattr(pixels, "dtype") or not hasattr(pixels, "shape"):
+                raise TypeError(
+                    f"Render method returns a {pixels.__class__.__name__}, but an array with dtype and shape is expected."
+                    "Be sure to specify the correct render_mode."
+                )
+
             if np.issubdtype(pixels.dtype, np.integer):
                 low, high = (0, 255)
             elif np.issubdtype(pixels.dtype, np.float):