Stop VecNormalize rewriting the observation space of the env it wraps

docs/misc/changelog.md

@@ -11,6 +11,7 @@
 ### Bug Fixes:
 
 - Fixed file descriptor leak in `SubprocVecEnv.close()` by closing parent-side pipes to prevent "too many open files" errors in long-running loops
+- Fixed `VecNormalize` writing the normalized image bounds into the `Dict` observation space of the environment it wraps, which changed how a model built on that environment afterwards treated the image
 
 ### [SB3-Contrib]
 

stable_baselines3/common/vec_env/vec_normalize.py

@@ -54,6 +54,8 @@ def __init__(
             self._sanity_checks()
 
             if isinstance(self.observation_space, spaces.Dict):
+                # Copy first: the image bounds below are written into this space, which the wrapped env owns
+                self.observation_space = deepcopy(self.observation_space)
                 self.obs_spaces = self.observation_space.spaces
                 self.obs_rms = {key: RunningMeanStd(shape=self.obs_spaces[key].shape) for key in self.norm_obs_keys}  # type: ignore[arg-type, union-attr]
                 # Update observation space when using image

tests/test_vec_normalize.py

@@ -1,4 +1,5 @@
 import operator
+from copy import deepcopy
 from typing import Any
 
 import gymnasium as gym
@@ -7,7 +8,7 @@
 from gymnasium import spaces
 
 from stable_baselines3 import SAC, TD3, HerReplayBuffer
-from stable_baselines3.common.envs import FakeImageEnv
+from stable_baselines3.common.envs import FakeImageEnv, SimpleMultiObsEnv
 from stable_baselines3.common.monitor import Monitor
 from stable_baselines3.common.running_mean_std import RunningMeanStd
 from stable_baselines3.common.vec_env import (
@@ -496,3 +497,13 @@ def test_non_dict_obs_keys():
 
     # Test dict obs with norm_obs set to False
     _make_warmstart(lambda: DummyMixedDictEnv(), norm_obs=False)
+
+
+def test_vec_normalize_keeps_the_wrapped_obs_space():
+    venv = DummyVecEnv([lambda: SimpleMultiObsEnv()])
+    original_space = deepcopy(venv.observation_space)
+
+    vec_normalize = VecNormalize(venv)
+
+    assert venv.observation_space == original_space
+    assert vec_normalize.observation_space["img"].dtype == np.float32