diff --git a/.gitignore b/.gitignore
index b1ec891..6cc5860 100644
--- a/.gitignore
+++ b/.gitignore
@@ -132,3 +132,6 @@ dmypy.json
 
 # pytype static type analyzer
 .pytype/
+
+# customize
+flake8.sh
diff --git a/flake8.sh b/flake8.sh
deleted file mode 100755
index 5f3cff1..0000000
--- a/flake8.sh
+++ /dev/null
@@ -1,3 +0,0 @@
-#!/bin/sh
-flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics                 
-flake8 . --count --exit-zero --max-complexity=20 --max-line-length=79 --statistics
diff --git a/tianshou/__init__.py b/tianshou/__init__.py
index 3e97de1..38edeca 100644
--- a/tianshou/__init__.py
+++ b/tianshou/__init__.py
@@ -2,8 +2,8 @@ from tianshou import data, env, utils, policy
 
 __version__ = '0.2.0'
 __all__ = [
-    'data',
     'env',
+    'data',
     'utils',
     'policy'
 ]
diff --git a/tianshou/data/batch.py b/tianshou/data/batch.py
index d254217..7dd2ca8 100644
--- a/tianshou/data/batch.py
+++ b/tianshou/data/batch.py
@@ -25,5 +25,5 @@ class Batch(object):
                 self.__dict__[k] += batch.__dict__[k]
             else:
                 raise TypeError(
-                    'No support append method with {} in class Batch.'
+                    'Do not support append with type {} in class Batch.'
                     .format(type(batch.__dict__[k])))
diff --git a/tianshou/data/collector.py b/tianshou/data/collector.py
index 8378d53..c5da90c 100644
--- a/tianshou/data/collector.py
+++ b/tianshou/data/collector.py
@@ -21,7 +21,7 @@ class Collector(object):
             self.env_num = len(env)
             if isinstance(self.buffer, list):
                 assert len(self.buffer) == self.env_num,\
-                    'Data buffer number does not match the input env number.'
+                    '# of data buffer does not match the # of input env.'
             elif isinstance(self.buffer, ReplayBuffer):
                 self.buffer = [deepcopy(buffer) for _ in range(self.env_num)]
             else:
diff --git a/tianshou/env/wrapper.py b/tianshou/env/wrapper.py
index 5a4fbb5..bef3aab 100644
--- a/tianshou/env/wrapper.py
+++ b/tianshou/env/wrapper.py
@@ -58,18 +58,18 @@ class FrameStack(EnvWrapper):
 
 
 class BaseVectorEnv(ABC):
-    def __init__(self):
-        pass
+    def __init__(self, env_fns, reset_after_done):
+        self._env_fns = env_fns
+        self.env_num = len(env_fns)
+        self._reset_after_done = reset_after_done
 
 
 class VectorEnv(BaseVectorEnv):
     """docstring for VectorEnv"""
 
     def __init__(self, env_fns, reset_after_done=False):
-        super().__init__()
+        super().__init__(env_fns, reset_after_done)
         self.envs = [_() for _ in env_fns]
-        self.env_num = len(self.envs)
-        self._reset_after_done = reset_after_done
 
     def __len__(self):
         return len(self.envs)
@@ -132,8 +132,7 @@ class SubprocVectorEnv(BaseVectorEnv):
     """docstring for SubProcVectorEnv"""
 
     def __init__(self, env_fns, reset_after_done=False):
-        super().__init__()
-        self.env_num = len(env_fns)
+        super().__init__(env_fns, reset_after_done)
         self.closed = False
         self.parent_remote, self.child_remote = \
             zip(*[Pipe() for _ in range(self.env_num)])
@@ -193,9 +192,7 @@ class RayVectorEnv(BaseVectorEnv):
     """docstring for RayVectorEnv"""
 
     def __init__(self, env_fns, reset_after_done=False):
-        super().__init__()
-        self.env_num = len(env_fns)
-        self._reset_after_done = reset_after_done
+        super().__init__(env_fns, reset_after_done)
         try:
             if not ray.is_initialized():
                 ray.init()
diff --git a/tianshou/policy/__init__.py b/tianshou/policy/__init__.py
index d7a3fb5..37c8423 100644
--- a/tianshou/policy/__init__.py
+++ b/tianshou/policy/__init__.py
@@ -1,5 +1,7 @@
 from tianshou.policy.base import BasePolicy
+from tianshou.policy.dqn import DQNPolicy
 
 __all__ = [
-    'BasePolicy'
+    'BasePolicy',
+    'DQNPolicy',
 ]
diff --git a/tianshou/policy/base.py b/tianshou/policy/base.py
index de2eb9f..afbeacd 100644
--- a/tianshou/policy/base.py
+++ b/tianshou/policy/base.py
@@ -9,7 +9,7 @@ class BasePolicy(ABC):
 
     @abstractmethod
     def act(self, batch, hidden_state=None):
-        # return {policy, action, hidden}
+        # return Batch(policy, action, hidden)
         pass
 
     def train(self):
diff --git a/tianshou/policy/reward_processor.py b/tianshou/policy/reward_processor.py
deleted file mode 100644
index e69de29..0000000