Add warning for Discrete action space with non-zero (DLR-RM#1295)

stable_baselines3/common/env_checker.py

@@ -93,6 +93,12 @@ def _check_unsupported_spaces(env: gym.Env, observation_space: spaces.Space, act
             "You can use a wrapper or update your observation space."
         )
 
+    if isinstance(action_space, spaces.Discrete) and action_space.start != 0:
+        warnings.warn(
+            "Discrete action space with a non-zero start is not supported by Stable-Baselines3. "
+            "You can use a wrapper or update your action 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

@@ -160,6 +160,8 @@ def patched_step(_action):
         spaces.Box(low=-np.inf, high=1, shape=(2,), dtype=np.float32),
         # Almost good, except for one dim
         spaces.Box(low=np.array([-1, -1, -1]), high=np.array([1, 1, 0.99]), dtype=np.float32),
+        # Non zero start index
+        spaces.Discrete(3, start=-1),
     ],
 )
 def test_non_default_action_spaces(new_action_space):
@@ -174,6 +176,12 @@ def test_non_default_action_spaces(new_action_space):
     # Change the action space
     env.action_space = new_action_space
 
+    # Discrete action space
+    if isinstance(new_action_space, spaces.Discrete):
+        with pytest.warns(UserWarning):
+            check_env(env)
+        return
+
     low, high = new_action_space.low[0], new_action_space.high[0]
     # Unbounded action space throws an error,
     # the rest only warning