Return output from render method in a right way (#1248)

* Close output StringIO after returning value

* Test render output is immutable

gym/envs/algorithmic/algorithmic_env.py

@@ -33,10 +33,11 @@ leveling up many times to reach their reward threshold.
 from gym import Env, logger
 from gym.spaces import Discrete, Tuple
 from gym.utils import colorize, seeding
+import sys
+from contextlib import closing
 import numpy as np
 from six import StringIO
-import sys
-import math
+
 
 class AlgorithmicEnv(Env):
 
@@ -112,7 +113,6 @@ class AlgorithmicEnv(Env):
         raise NotImplementedError
 
     def render(self, mode='human'):
-
         outfile = StringIO() if mode == 'ansi' else sys.stdout
         inp = "Total length of input instance: %d, step: %d\n" % (self.input_width, self.time)
         outfile.write(inp)
@@ -149,7 +149,10 @@ class AlgorithmicEnv(Env):
             outfile.write("                              prediction: %s)\n" % pred_str)
         else:
             outfile.write("\n" * 5)
-        return outfile
+
+        if mode != 'human':
+            with closing(outfile):
+                return outfile.getvalue()
 
     @property
     def input_width(self):
@@ -234,6 +237,7 @@ class AlgorithmicEnv(Env):
     def _move(self, movement):
         raise NotImplemented
 
+
 class TapeAlgorithmicEnv(AlgorithmicEnv):
     """An algorithmic env with a 1-d input tape."""
     MOVEMENTS = ['left', 'right']
@@ -269,6 +273,7 @@ class TapeAlgorithmicEnv(AlgorithmicEnv):
         x_str += "\n"
         return x_str
 
+
 class GridAlgorithmicEnv(AlgorithmicEnv):
     """An algorithmic env with a 2-d input grid."""
     MOVEMENTS = ['left', 'right', 'up', 'down']

gym/envs/tests/test_envs.py

@@ -48,3 +48,17 @@ def test_random_rollout():
             (ob, _reward, done, _info) = env.step(a)
             if done: break
         env.close()
+
+
+def test_env_render_result_is_immutable():
+    environs = [
+        envs.make('Taxi-v2'),
+        envs.make('FrozenLake-v0'),
+        envs.make('Reverse-v0'),
+    ]
+
+    for env in environs:
+        env.reset()
+        output = env.render(mode='ansi')
+        assert isinstance(output, str)
+        env.close()

gym/envs/toy_text/frozen_lake.py

@@ -1,5 +1,7 @@
-import numpy as np
 import sys
+from contextlib import closing
+
+import numpy as np
 from six import StringIO, b
 
 from gym import utils
@@ -129,4 +131,5 @@ class FrozenLakeEnv(discrete.DiscreteEnv):
         outfile.write("\n".join(''.join(line) for line in desc)+"\n")
 
         if mode != 'human':
-            return outfile
+            with closing(outfile):
+                return outfile.getvalue()

gym/envs/toy_text/taxi.py

@@ -1,4 +1,5 @@
 import sys
+from contextlib import closing
 from six import StringIO
 from gym import utils
 from gym.envs.toy_text import discrete
@@ -149,4 +150,5 @@ class TaxiEnv(discrete.DiscreteEnv):
 
         # No need to return anything for human
         if mode != 'human':
-            return outfile
+            with closing(outfile):
+                return outfile.getvalue()