[MLA-1172] Reduce calls to training_behaviors (#4259)

com.unity.ml-agents/CHANGELOG.md

 ### Major Changes
 #### com.unity.ml-agents (C#)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
-The minimum supported python version for ml-agents-envs was changed to 3.6.1. (#4244)
+- The minimum supported python version for ml-agents-envs was changed to 3.6.1. (#4244)
+- The interaction between EnvManager and TrainerController was changed; EnvManager.advance() was split into to stages,
+and TrainerController now uses the results from the first stage to handle new behavior names. This change speeds up
+Python training by approximately 5-10%. (#4259)
 
 ### Minor Changes
 #### com.unity.ml-agents (C#)

ml-agents/mlagents/trainers/env_manager.py

     def __init__(self):
         self.policies: Dict[BehaviorName, TFPolicy] = {}
         self.agent_managers: Dict[BehaviorName, AgentManager] = {}
-        self.first_step_infos: List[EnvironmentStep] = None
+        self.first_step_infos: List[EnvironmentStep] = []
 
     def set_policy(self, brain_name: BehaviorName, policy: TFPolicy) -> None:
         self.policies[brain_name] = policy
     def close(self):
         pass
 
-    def advance(self):
+    def get_steps(self) -> List[EnvironmentStep]:
+        """
+        Updates the policies, steps the environments, and returns the step information from the environments.
+        Calling code should pass the returned EnvironmentSteps to process_steps() after calling this.
+        :return: The list of EnvironmentSteps
+        """
-        if self.first_step_infos is not None:
+        if self.first_step_infos:
-            self.first_step_infos = None
+            self.first_step_infos = []
-        for brain_name in self.training_behaviors:
+        for brain_name in self.agent_managers.keys():
             _policy = None
             try:
                 # We make sure to empty the policy queue before continuing to produce steps.
             except AgentManagerQueue.Empty:
                 if _policy is not None:
-                    self.set_policy(brain_name, _policy)
-        # Step the environment
+                    # policy_queue contains Policy, but we need a TFPolicy here
+                    self.set_policy(brain_name, _policy)  # type: ignore
+        # Step the environments
+        return new_step_infos
+
+    def process_steps(self, new_step_infos: List[EnvironmentStep]) -> int:
         # Add to AgentProcessor
         num_step_infos = self._process_step_infos(new_step_infos)
         return num_step_infos

ml-agents/mlagents/trainers/tests/test_subprocess_env_manager.py

         }
         step_info = EnvironmentStep(step_info_dict, 0, action_info_dict, env_stats)
         step_mock.return_value = [step_info]
-        env_manager.advance()
+        env_manager.process_steps(env_manager.get_steps())
 
         # Test add_experiences
         env_manager._step.assert_called_once()

ml-agents/mlagents/trainers/tests/test_trainer_controller.py

     tc.advance(env_mock)
 
     env_mock.reset.assert_not_called()
-    env_mock.advance.assert_called_once()
+    env_mock.get_steps.assert_called_once()
+    env_mock.process_steps.assert_called_once()
     # May have been called many times due to thread
     trainer_mock.advance.call_count > 0

ml-agents/mlagents/trainers/trainer_controller.py

 from mlagents.tf_utils import tf
 
 from mlagents_envs.logging_util import get_logger
-from mlagents.trainers.env_manager import EnvManager
+from mlagents.trainers.env_manager import EnvManager, EnvironmentStep
 from mlagents_envs.exception import (
     UnityEnvironmentException,
     UnityCommunicationException,
         self.train_model = train
         self.param_manager = param_manager
         self.ghost_controller = self.trainer_factory.ghost_controller
+        self.registered_behavior_ids: Set[str] = set()
 
         self.trainer_threads: List[threading.Thread] = []
         self.kill_trainers = False
             )
 
     @timed
-    def _reset_env(self, env: EnvManager) -> None:
+    def _reset_env(self, env_manager: EnvManager) -> None:
         """Resets the environment.
 
         Returns:
         new_config = self.param_manager.get_current_samplers()
-        env.reset(config=new_config)
+        env_manager.reset(config=new_config)
+        # Register any new behavior ids that were generated on the reset.
+        self._register_new_behaviors(env_manager, env_manager.first_step_infos)
 
     def _not_done_training(self) -> bool:
         return (
     def start_learning(self, env_manager: EnvManager) -> None:
         self._create_output_path(self.output_path)
         tf.reset_default_graph()
-        last_brain_behavior_ids: Set[str] = set()
-                external_brain_behavior_ids = set(env_manager.training_behaviors.keys())
-                new_behavior_ids = external_brain_behavior_ids - last_brain_behavior_ids
-                self._create_trainers_and_managers(env_manager, new_behavior_ids)
-                last_brain_behavior_ids = external_brain_behavior_ids
                 n_steps = self.advance(env_manager)
                 for _ in range(n_steps):
                     self.reset_env_if_ready(env_manager)
             env.set_env_parameters(self.param_manager.get_current_samplers())
 
     @timed
-    def advance(self, env: EnvManager) -> int:
+    def advance(self, env_manager: EnvManager) -> int:
-            num_steps = env.advance()
+            new_step_infos = env_manager.get_steps()
+            self._register_new_behaviors(env_manager, new_step_infos)
+            num_steps = env_manager.process_steps(new_step_infos)
 
         # Report current lesson for each environment parameter
         for (
                     trainer.advance()
 
         return num_steps
+
+    def _register_new_behaviors(
+        self, env_manager: EnvManager, step_infos: List[EnvironmentStep]
+    ) -> None:
+        """
+        Handle registration (adding trainers and managers) of new behaviors ids.
+        :param env_manager:
+        :param step_infos:
+        :return:
+        """
+        step_behavior_ids: Set[str] = set()
+        for s in step_infos:
+            step_behavior_ids |= set(s.name_behavior_ids)
+        new_behavior_ids = step_behavior_ids - self.registered_behavior_ids
+        self._create_trainers_and_managers(env_manager, new_behavior_ids)
+        self.registered_behavior_ids |= step_behavior_ids
 
     def join_threads(self, timeout_seconds: float = 1.0) -> None:
         """