Fix Framestack obs env_checker

docs/misc/changelog.rst

@@ -99,6 +99,7 @@ Bug Fixes:
   for ``Inf`` and ``NaN`` (@lutogniew)
 - Fixed HER ``truncate_last_trajectory()`` (@lbergmann1)
 - Fixed HER desired and achieved goal order in reward computation (@JonathanKuelz)
+- Fixed ``env_checker`` for ``FrameStack`` observation (@corentinlger)
 
 Deprecations:
 ^^^^^^^^^^^^^
@@ -1410,4 +1411,4 @@ And all the contributors:
 @Melanol @qgallouedec @francescoluciano @jlp-ue @burakdmb @timothe-chaumont @honglu2875
 @anand-bala @hughperkins @sidney-tio @AlexPasqua @dominicgkerr @Akhilez @Rocamonde @tobirohrer @ZikangXiong
 @DavyMorgan @luizapozzobon @Bonifatius94 @theSquaredError @harveybellini @DavyMorgan @FieteO @jonasreiher @npit @WeberSamuel @troiganto
-@lutogniew @lbergmann1 @lukashass @BertrandDecoster @pseudo-rnd-thoughts @stefanbschneider @kyle-he
+@lutogniew @lbergmann1 @lukashass @BertrandDecoster @pseudo-rnd-thoughts @stefanbschneider @kyle-he @corentinlger

stable_baselines3/common/env_checker.py

@@ -4,6 +4,7 @@
 import gymnasium as gym
 import numpy as np
 from gymnasium import spaces
+from gymnasium.wrappers.frame_stack import LazyFrames
 
 from stable_baselines3.common.preprocessing import check_for_nested_spaces, is_image_space_channels_first
 from stable_baselines3.common.vec_env import DummyVecEnv, VecCheckNan
@@ -171,7 +172,9 @@ def _check_goal_env_compute_reward(
     assert rewards[0] == reward, f"Vectorized computation of reward differs from single computation: {rewards[0]} != {reward}"
 
 
-def _check_obs(obs: Union[tuple, dict, np.ndarray, int], observation_space: spaces.Space, method_name: str) -> None:
+def _check_obs(
+    obs: Union[tuple, dict, np.ndarray, int, LazyFrames], observation_space: spaces.Space, method_name: str
+) -> None:
     """
     Check that the observation returned by the environment
     correspond to the declared one.
@@ -187,7 +190,10 @@ def _check_obs(obs: Union[tuple, dict, np.ndarray, int], observation_space: spac
         # `sample()` will return a np.int64 instead of an int
         assert np.issubdtype(type(obs), np.integer), f"The observation returned by `{method_name}()` method must be an int"
     elif _is_numpy_array_space(observation_space):
-        assert isinstance(obs, np.ndarray), f"The observation returned by `{method_name}()` method must be a numpy array"
+        # Check if obs is a ndarray or a FrameStacking of ndarrays
+        assert isinstance(
+            obs, (np.ndarray, LazyFrames)
+        ), f"The observation returned by `{method_name}()` method must be a numpy array"
 
     # Additional checks for numpy arrays, so the error message is clearer (see GH#1399)
     if isinstance(obs, np.ndarray):