[Feature] auto_wrap_envs in PEnv

test/conftest.py

@@ -74,6 +74,11 @@ def set_warnings() -> None:
         category=UserWarning,
         message=r"Skipping device Apple Paravirtual device",
     )
+    warnings.filterwarnings(
+        "ignore",
+        category=UserWarning,
+        message=r"A lambda function was passed to ParallelEnv",
+    )
     warnings.filterwarnings(
         "ignore",
         category=DeprecationWarning,

test/test_envs.py

@@ -9,7 +9,7 @@
 import functools
 import gc
 import importlib
-import os.path
+import os
 import pickle
 import random
 import re

torchrl/envs/batched_envs.py

@@ -37,6 +37,7 @@
     _check_for_faulty_process,
     _make_ordinal_device,
     logger as torchrl_logger,
+    rl_warnings,
     VERBOSE,
 )
 from torchrl.data.tensor_specs import Composite, NonTensor
@@ -152,29 +153,50 @@ def __call__(cls, *args, **kwargs):
         # Wrap lambda functions with EnvCreator so they can be pickled for
         # multiprocessing with the spawn start method. Lambda functions cannot
         # be serialized with standard pickle, but EnvCreator uses cloudpickle.
+        auto_wrap_envs = kwargs.pop("auto_wrap_envs", True)
+
+        def _warn_lambda():
+            if rl_warnings():
+                warnings.warn(
+                    "A lambda function was passed to ParallelEnv and will be wrapped "
+                    "in an EnvCreator. This causes the environment to be instantiated "
+                    "in the main process to extract metadata. Consider using "
+                    "functools.partial instead, which is natively serializable and "
+                    "avoids this overhead. To suppress this warning, set the "
+                    "RL_WARNINGS=0 environment variable.",
+                    category=UserWarning,
+                    stacklevel=4,
+                )
+
         def _wrap_lambdas(create_env_fn):
             if callable(create_env_fn) and _is_unpicklable_lambda(create_env_fn):
+                _warn_lambda()
                 return EnvCreator(create_env_fn)
             if isinstance(create_env_fn, Sequence):
                 # Reuse EnvCreator for identical function objects to preserve
                 # _single_task detection (e.g., when [lambda_fn] * 3 is passed)
                 wrapped = {}
                 result = []
+                warned = False
                 for fn in create_env_fn:
                     if _is_unpicklable_lambda(fn):
                         fn_id = id(fn)
                         if fn_id not in wrapped:
+                            if not warned:
+                                _warn_lambda()
+                                warned = True
                             wrapped[fn_id] = EnvCreator(fn)
                         result.append(wrapped[fn_id])
                     else:
                         result.append(fn)
                 return result
             return create_env_fn
 
-        if "create_env_fn" in kwargs:
-            kwargs["create_env_fn"] = _wrap_lambdas(kwargs["create_env_fn"])
-        elif len(args) >= 2:
-            args = (args[0], _wrap_lambdas(args[1])) + args[2:]
+        if auto_wrap_envs:
+            if "create_env_fn" in kwargs:
+                kwargs["create_env_fn"] = _wrap_lambdas(kwargs["create_env_fn"])
+            elif len(args) >= 2:
+                args = (args[0], _wrap_lambdas(args[1])) + args[2:]
 
         return super().__call__(*args, **kwargs)
 
@@ -241,6 +263,20 @@ class BatchedEnvBase(EnvBase):
         daemon (bool, optional): whether the processes should be daemonized.
             This is only applicable to parallel environments such as :class:`~torchrl.envs.ParallelEnv`.
             Defaults to ``False``.
+        auto_wrap_envs (bool, optional): if ``True`` (default), lambda functions passed as
+            ``create_env_fn`` will be automatically wrapped in an :class:`~torchrl.envs.EnvCreator`
+            to enable pickling for multiprocessing with the ``spawn`` start method.
+            This wrapping causes the environment to be instantiated once in the main process
+            (to extract metadata) before workers are started.
+            If this is undesirable, set ``auto_wrap_envs=False``. Otherwise, ensure your callable is
+            serializable (e.g., use :func:`functools.partial` instead of lambdas).
+            This parameter only affects :class:`~torchrl.envs.ParallelEnv`.
+            Defaults to ``True``.
+
+    .. note::
+        For :class:`~torchrl.envs.ParallelEnv`, it is recommended to use :func:`functools.partial`
+        instead of lambda functions when possible, as ``partial`` objects are natively serializable
+        and avoid the overhead of :class:`~torchrl.envs.EnvCreator` wrapping.
 
     .. note::
         One can pass keyword arguments to each sub-environments using the following