[Feature, Test] Adding tests for envs that have no specs

test/mocking_classes.py

@@ -1996,3 +1996,17 @@ def _step(
 
     def _set_seed(self, seed: Optional[int]):
         ...
+
+
+class EnvThatDoesNothing(EnvBase):
+    def _reset(self, tensordict: TensorDictBase, **kwargs) -> TensorDictBase:
+        return TensorDict(batch_size=self.batch_size, device=self.device)
+
+    def _step(
+        self,
+        tensordict: TensorDictBase,
+    ) -> TensorDictBase:
+        return TensorDict(batch_size=self.batch_size, device=self.device)
+
+    def _set_seed(self, seed):
+        ...

test/test_env.py

@@ -44,6 +44,7 @@
         DiscreteActionConvMockEnvNumpy,
         DiscreteActionVecMockEnv,
         DummyModelBasedEnvBase,
+        EnvThatDoesNothing,
         EnvWithDynamicSpec,
         EnvWithMetadata,
         HeterogeneousCountingEnv,
@@ -81,6 +82,7 @@
         DiscreteActionConvMockEnvNumpy,
         DiscreteActionVecMockEnv,
         DummyModelBasedEnvBase,
+        EnvThatDoesNothing,
         EnvWithDynamicSpec,
         EnvWithMetadata,
         HeterogeneousCountingEnv,
@@ -3554,6 +3556,34 @@ def test_auto_spec():
     env.check_env_specs(tensordict=td.copy())
 
 
+def test_env_that_does_nothing():
+    env = EnvThatDoesNothing()
+    env.check_env_specs()
+    r = env.rollout(3)
+    r.exclude(
+        "done", "terminated", ("next", "done"), ("next", "terminated"), inplace=True
+    )
+    assert r.is_empty()
+    p_env = SerialEnv(2, EnvThatDoesNothing)
+    p_env.check_env_specs()
+    r = p_env.rollout(3)
+    r.exclude(
+        "done", "terminated", ("next", "done"), ("next", "terminated"), inplace=True
+    )
+    assert r.is_empty()
+    p_env = ParallelEnv(2, EnvThatDoesNothing)
+    try:
+        p_env.check_env_specs()
+        r = p_env.rollout(3)
+        r.exclude(
+            "done", "terminated", ("next", "done"), ("next", "terminated"), inplace=True
+        )
+        assert r.is_empty()
+    finally:
+        p_env.close()
+        del p_env
+
+
 if __name__ == "__main__":
     args, unknown = argparse.ArgumentParser().parse_known_args()
     pytest.main([__file__, "--capture", "no", "--exitfirst"] + unknown)

torchrl/envs/common.py

@@ -2434,8 +2434,12 @@ def _register_gym(  # noqa: F811
             apply_api_compatibility=apply_api_compatibility,
         )
 
-    def forward(self, tensordict: TensorDictBase) -> TensorDictBase:
-        raise NotImplementedError("EnvBase.forward is not implemented")
+    def forward(self, *args, **kwargs):
+        raise NotImplementedError(
+            "EnvBase.forward is not implemented. If you ended here during a call to `ParallelEnv(...)`, please use "
+            "a constructor such as `ParallelEnv(num_env, lambda env=env: env)` instead. "
+            "Batched envs require constructors because environment instances may not always be serializable."
+        )
 
     @abc.abstractmethod
     def _step(

torchrl/envs/utils.py

@@ -287,6 +287,8 @@ def __call__(self, tensordict):
         if self.validate(tensordict):
             if self.keep_other:
                 out = self._exclude(self.exclude_from_root, tensordict, out=None)
+                if out is None:
+                    out = tensordict.empty()
             else:
                 out = next_td.empty()
                 self._grab_and_place(