Add warning when using non-zero start with Discrete (fixes DLR-RM#1197)

stable_baselines3/common/env_checker.py

@@ -56,9 +56,15 @@ def _check_unsupported_spaces(env: gym.Env, observation_space: spaces.Space, act
 
     if isinstance(observation_space, spaces.Dict):
         nested_dict = False
-        for space in observation_space.spaces.values():
+        for key, space in observation_space.spaces.items():
             if isinstance(space, spaces.Dict):
                 nested_dict = True
+            if isinstance(space, spaces.Discrete) and space.start != 0:
+                warnings.warn(
+                    f"Discrete observation space (key '{key}') with a non-zero start is not supported by Stable-Baselines3. "
+                    "You can use a wrapper or update your observation space."
+                )
+
         if nested_dict:
             warnings.warn(
                 "Nested observation spaces are not supported by Stable Baselines3 "
@@ -77,6 +83,12 @@ def _check_unsupported_spaces(env: gym.Env, observation_space: spaces.Space, act
             "which is supported by SB3."
         )
 
+    if isinstance(observation_space, spaces.Discrete) and observation_space.start != 0:
+        warnings.warn(
+            "Discrete observation space with a non-zero start is not supported by Stable-Baselines3. "
+            "You can use a wrapper or update your observation space."
+        )
+
     if not _is_numpy_array_space(action_space):
         warnings.warn(
             "The action space is not based off a numpy array. Typically this means it's either a Dict or Tuple space. "

tests/test_envs.py

@@ -121,6 +121,10 @@ def patched_step(_action):
         spaces.Dict({"position": spaces.Dict({"abs": spaces.Discrete(5), "rel": spaces.Discrete(2)})}),
         # Small image inside a dict
         spaces.Dict({"img": spaces.Box(low=0, high=255, shape=(32, 32, 3), dtype=np.uint8)}),
+        # Non zero start index
+        spaces.Discrete(3, start=-1),
+        # Non zero start index inside a Dict
+        spaces.Dict({"obs": spaces.Discrete(3, start=1)}),
     ],
 )
 def test_non_default_spaces(new_obs_space):