Tweak Wrapper implementation (openai#291)

* Tweak README and don't use __new__ for wrapper

The extra 'env' argument means that Wrapper authors with a custom
__init__ need to think about the parent signature anyway, so there's
not as much benefit to adding the more surprising __new__.

* Add support for registering wrappers for the official environment

gym/core.py

@@ -240,7 +240,7 @@ def configure(self, *args, **kwargs):
             else:
                 raise
 
-    def build(self):
+    def build(self, extra_wrappers=None):
         """[EXPERIMENTAL: may be removed in a later version of Gym] Builds an
         environment by applying any provided wrappers, with the
         outmost wrapper supplied first. This method is automatically
@@ -259,12 +259,19 @@ class FooEnv(gym.Env):
 
             Calling 'env.build' will return 'Wrapper1(Wrapper2(env))'.
 
+        Args:
+            extra_wrappers (Optional[list]): Any extra wrappers to apply to the wrapped instance
+
         Returns:
             gym.Env: A potentially wrapped environment instance.
 
         """
+        wrappers = self.metadata.get('wrappers', [])
+        if extra_wrappers:
+            wrappers = wrappers + extra_wrappers
+
         wrapped = self
-        for wrapper in reversed(self.metadata.get('wrappers', [])):
+        for wrapper in reversed(wrappers):
             wrapped = wrapper(wrapped)
         return wrapped
 
@@ -314,23 +321,14 @@ def from_jsonable(self, sample_n):
         return sample_n
 
 class Wrapper(Env):
-
-    def __new__(cls, env, *args, **kwargs):
-        # We use __new__ since we want the wrapper author to be able to
-        # override __init__ without remembering to call super.
-        wrapper = super(Wrapper, cls).__new__(cls)
-        wrapper.env = env
-        wrapper.metadata = env.metadata
-        wrapper.action_space = env.action_space
-        wrapper.observation_space = env.observation_space
-        wrapper.reward_range = env.reward_range
-        wrapper.spec = env.spec
-        wrapper._unwrapped = env.unwrapped
-        return wrapper
-
-    # Overridable
     def __init__(self, env):
-        pass
+        self.env = env
+        self.metadata = env.metadata
+        self.action_space = env.action_space
+        self.observation_space = env.observation_space
+        self.reward_range = env.reward_range
+        self.spec = env.spec
+        self._unwrapped = env.unwrapped
 
     def _step(self, action):
         return self.env.step(action)

gym/envs/registration.py

@@ -34,7 +34,7 @@ class EnvSpec(object):
         trials (int): The number of trials run in official evaluation
     """
 
-    def __init__(self, id, entry_point=None, timestep_limit=1000, trials=100, reward_threshold=None, local_only=False, kwargs=None, nondeterministic=False):
+    def __init__(self, id, entry_point=None, timestep_limit=1000, trials=100, reward_threshold=None, local_only=False, kwargs=None, nondeterministic=False, wrappers=None):
         self.id = id
         # Evaluation parameters
         self.timestep_limit = timestep_limit
@@ -52,6 +52,7 @@ def __init__(self, id, entry_point=None, timestep_limit=1000, trials=100, reward
         self._entry_point = entry_point
         self._local_only = local_only
         self._kwargs = {} if kwargs is None else kwargs
+        self._wrappers = wrappers
 
     def make(self):
         """Instantiates an instance of the environment with appropriate kwargs"""
@@ -63,7 +64,7 @@ def make(self):
 
         # Make the enviroment aware of which spec it came from.
         env.spec = self
-        env = env.build()
+        env = env.build(extra_wrappers=self._wrappers)
 
         return env
 

gym/wrappers/README.md

@@ -1,9 +1,12 @@
-# Wrappers
+# Wrappers (experimental)
 
-These are the wrappers to customize environments. Note that we may later
-restructure any of the files, but will keep the wrappers available
-at the relevant package's top-level. So for example, you should access
-`MyWrapper` as follows:
+This is a placeholder for now: we will likely soon start adding
+standardized wrappers for environments. (Only stable and
+general-purpose wrappers will be accepted into gym core.)
+
+Note that we may later restructure any of the files, but will keep the
+wrappers available at the relevant package's top-level. So for
+example, you should access `MyWrapper` as follows:
 
 ```
 # Will be supported in future releases