Suppresses AssertionError when using InteractiveSession.

The following code would previously raise an AssertionError in the
garbage collector:

```python
sess = tf.InteractiveSession()
sess = tf.InteractiveSession()
```

This happens because the second session will be installed on the
default session stack before the first session is destroyed, which
leads to the assertion (of strict nesting) failing. Since this is
common behavior in IPython notebooks, we suppress this error for
InteractiveSession objects.

Fixes #2474.
Change: 123040755

tensorflow/python/client/session.py

@@ -900,10 +900,12 @@ class InteractiveSession(BaseSession):
 
     super(InteractiveSession, self).__init__(target, graph, config)
     self._default_session = self.as_default()
+    self._default_session.enforce_nesting = False
     self._default_session.__enter__()
     self._explicit_graph = graph
     if self._explicit_graph is not None:
       self._default_graph = graph.as_default()
+      self._default_graph.enforce_nesting = False
       self._default_graph.__enter__()
 
   def close(self):

tensorflow/python/client/session_test.py

@@ -1138,11 +1138,33 @@ class SessionTest(test_util.TensorFlowTestCase):
         d = math_ops.mul(c, c)
       for step in xrange(120):
         run_metadata = config_pb2.RunMetadata()
-        sess.run(d, feed_dict={a: 1.0}, options=run_options, run_metadata=run_metadata)
+        sess.run(d, feed_dict={a: 1.0},
+                 options=run_options, run_metadata=run_metadata)
         if step == 99:
           self.assertTrue(run_metadata.HasField('cost_graph'))
         else:
           self.assertFalse(run_metadata.HasField('cost_graph'))
 
+  def testNonInteractiveSessionNesting(self):
+    sess1 = session.Session()
+    sess1_controller = sess1.as_default()
+    sess1_controller.__enter__()
+
+    sess2 = session.Session()
+    sess2_controller = sess2.as_default()
+    sess2_controller.__enter__()
+
+    with self.assertRaisesRegexp(AssertionError, 'Nesting violated'):
+      sess1_controller.__exit__(None, None, None)
+
+    ops._default_session_stack.reset()
+
+  def testInteractiveSessionNesting(self):
+    sess1 = session.InteractiveSession()
+    sess2 = session.InteractiveSession()
+    del sess1
+    del sess2
+
+
 if __name__ == '__main__':
   googletest.main()

tensorflow/python/framework/ops.py

@@ -3306,6 +3306,7 @@ class _DefaultStack(threading.local):
 
   def __init__(self):
     super(_DefaultStack, self).__init__()
+    self._enforce_nesting = True
     self.stack = []
 
   def get_default(self):
@@ -3314,6 +3315,14 @@ class _DefaultStack(threading.local):
   def reset(self):
     self.stack = []
 
+  @property
+  def enforce_nesting(self):
+    return self._enforce_nesting
+
+  @enforce_nesting.setter
+  def enforce_nesting(self, value):
+    self._enforce_nesting = value
+
   @contextlib.contextmanager
   def get_controller(self, default):
     """A context manager for manipulating a default stack."""
@@ -3321,9 +3330,14 @@ class _DefaultStack(threading.local):
       self.stack.append(default)
       yield default
     finally:
-      assert self.stack[-1] is default
-      self.stack.pop()
-
+      if self._enforce_nesting:
+        if self.stack[-1] is not default:
+          raise AssertionError(
+              "Nesting violated for default stack of %s objects"
+              % type(default))
+        self.stack.pop()
+      else:
+        self.stack.remove(default)
 
 _default_session_stack = _DefaultStack()