diff --git a/tensorflow/python/autograph/converters/BUILD b/tensorflow/python/autograph/converters/BUILD
index 9c1d5a38707..ec780a7c0a1 100644
--- a/tensorflow/python/autograph/converters/BUILD
+++ b/tensorflow/python/autograph/converters/BUILD
@@ -33,6 +33,7 @@ py_library(
         "logical_expressions.py",
         "return_statements.py",
         "slices.py",
+        "variables.py",
     ],
     srcs_version = "PY2AND3",
     visibility = ["//tensorflow:__subpackages__"],
@@ -213,3 +214,16 @@ py_test(
         "//tensorflow/python/autograph/pyct",
     ],
 )
+
+py_test(
+    name = "variables_test",
+    srcs = ["variables_test.py"],
+    python_version = "PY3",
+    srcs_version = "PY2AND3",
+    deps = [
+        ":converters",
+        "//tensorflow/python:client_testlib",
+        "//tensorflow/python/autograph/core:test_lib",
+        "//tensorflow/python/autograph/pyct",
+    ],
+)
diff --git a/tensorflow/python/autograph/converters/asserts_test.py b/tensorflow/python/autograph/converters/asserts_test.py
index fd31cd15a0e..dc435cbc90e 100644
--- a/tensorflow/python/autograph/converters/asserts_test.py
+++ b/tensorflow/python/autograph/converters/asserts_test.py
@@ -20,6 +20,7 @@ from __future__ import print_function
 
 from tensorflow.python.autograph.converters import asserts
 from tensorflow.python.autograph.converters import functions
+from tensorflow.python.autograph.converters import return_statements
 from tensorflow.python.autograph.core import converter_testing
 from tensorflow.python.framework import constant_op
 from tensorflow.python.framework import errors_impl
@@ -36,7 +37,8 @@ class AssertsTest(converter_testing.TestCase):
       return a
 
     with ops.Graph().as_default():
-      with self.converted(test_fn, (functions, asserts), {}) as result:
+      with self.converted(
+          test_fn, (functions, asserts, return_statements), {}) as result:
         op = result.test_fn(constant_op.constant(False))
 
       with self.assertRaisesRegexp(errors_impl.InvalidArgumentError, 'testmsg'):
diff --git a/tensorflow/python/autograph/converters/functions.py b/tensorflow/python/autograph/converters/functions.py
index fc33dafb63d..26ead131f9b 100644
--- a/tensorflow/python/autograph/converters/functions.py
+++ b/tensorflow/python/autograph/converters/functions.py
@@ -38,15 +38,6 @@ class _Function(object):
 class FunctionTransformer(converter.Base):
   """Wraps function bodies around autograph-specific boilerplate."""
 
-  def visit_Return(self, node):
-    if node.value is None:
-      return node
-    node = self.generic_visit(node)
-    return templates.replace(
-        'return function_context_name.mark_return_value(value)',
-        function_context_name=self.state[_Function].context_name,
-        value=node.value)
-
   def _function_scope_options(self, fn_scope):
     """Returns the options with which to create function scopes."""
     # Top-level function receive the options that were directly requested.
diff --git a/tensorflow/python/autograph/converters/functions_test.py b/tensorflow/python/autograph/converters/functions_test.py
index aad455e67d7..2a51ef71ebf 100644
--- a/tensorflow/python/autograph/converters/functions_test.py
+++ b/tensorflow/python/autograph/converters/functions_test.py
@@ -19,6 +19,7 @@ from __future__ import division
 from __future__ import print_function
 
 from tensorflow.python.autograph.converters import functions
+from tensorflow.python.autograph.converters import return_statements
 from tensorflow.python.autograph.core import ag_ctx
 from tensorflow.python.autograph.core import converter
 from tensorflow.python.autograph.core import converter_testing
@@ -74,7 +75,7 @@ class FunctionTransformer(converter_testing.TestCase):
       l += 1
       return l, inner_fn(l)
 
-    with self.converted(test_fn, functions, {},
+    with self.converted(test_fn, (functions, return_statements), {},
                         (ops.name_scope,)) as result:
       first, second = result.test_fn(constant_op.constant(1))
       self.assertIn('test_fn/', first.op.name)
@@ -119,6 +120,7 @@ class FunctionTransformer(converter_testing.TestCase):
     ns = {'TestClass': TestClass}
     node, ctx = self.prepare(TestClass, ns)
     node = functions.transform(node, ctx)
+    node = return_statements.transform(node, ctx)
 
     with self.compiled(node, {}, (ops.name_scope,)) as result:
       first, second = result.TestClass().test_fn(constant_op.constant(1))
diff --git a/tensorflow/python/autograph/converters/return_statements.py b/tensorflow/python/autograph/converters/return_statements.py
index 39bac60fb91..e4062e42db7 100644
--- a/tensorflow/python/autograph/converters/return_statements.py
+++ b/tensorflow/python/autograph/converters/return_statements.py
@@ -220,9 +220,9 @@ class ReturnStatementsTransformer(converter.Base):
         retval = val
   """
 
-  def __init__(self, ctx, default_to_null_return):
+  def __init__(self, ctx, allow_missing_return):
     super(ReturnStatementsTransformer, self).__init__(ctx)
-    self.default_to_null_return = default_to_null_return
+    self.allow_missing_return = allow_missing_return
 
   def visit_Return(self, node):
     for block in reversed(self.state[_Block].stack):
@@ -339,75 +339,68 @@ class ReturnStatementsTransformer(converter.Base):
     return node
 
   def visit_FunctionDef(self, node):
-    self.state[_Function].enter()
-    self.state[_Block].enter()
-    self.state[_Block].is_function = True
+    with self.state[_Function] as fn:
+      with self.state[_Block] as block:
+        block.is_function = True
 
-    scope = anno.getanno(node, NodeAnno.BODY_SCOPE)
-    do_return_var_name = self.ctx.namer.new_symbol(
-        'do_return', scope.referenced)
-    retval_var_name = self.ctx.namer.new_symbol('retval_', scope.referenced)
-    self.state[_Function].do_return_var_name = do_return_var_name
-    self.state[_Function].retval_var_name = retval_var_name
+        scope = anno.getanno(node, NodeAnno.BODY_SCOPE)
+        do_return_var_name = self.ctx.namer.new_symbol('do_return',
+                                                       scope.referenced)
+        retval_var_name = self.ctx.namer.new_symbol('retval_', scope.referenced)
+        fn.do_return_var_name = do_return_var_name
+        fn.retval_var_name = retval_var_name
 
-    converted_body = self._visit_statement_block(node, node.body)
+        node.body = self._visit_statement_block(node, node.body)
 
-    # Avoid placing statements before any eventual docstring.
-    # TODO(mdan): Should a docstring even be included in the output?
-    docstring = None
-    if converted_body:
-      if (isinstance(converted_body[0], gast.Expr) and
-          isinstance(converted_body[0].value, gast.Constant)):
-        docstring = converted_body[0]
-        converted_body = converted_body[1:]
+        if block.return_used:
 
-    if self.state[_Block].return_used:
+          if self.allow_missing_return:
+            # The function whould have a single `with` node that wraps the
+            # entire body. If the function had a docstring, the body has two
+            # nodes, with the `with` as the second node.
+            wrapper_node = node.body[-1]
+            assert isinstance(wrapper_node, gast.With), (
+                'This transformer requires the functions converter.')
 
-      if self.default_to_null_return:
-        # TODO(mdan): Remove the (do_return_var_name,) below.
-        # Currently, that line ensures the variable is both defined and alive
-        # throughout the function.
-        template = """
-          do_return_var_name = False
-          retval_var_name = ag__.UndefinedReturnValue()
-          body
-          (do_return_var_name,)
-          return ag__.retval(retval_var_name)
-        """
-      else:
-        template = """
-          body
-          return retval_var_name
-        """
-      node.body = templates.replace(
-          template,
-          body=converted_body,
-          do_return_var_name=do_return_var_name,
-          retval_var_name=retval_var_name)
+            template = """
+              do_return_var_name = False
+              retval_var_name = ag__.UndefinedReturnValue()
+              body
+              return function_context.ret(retval_var_name, do_return_var_name)
+            """
 
-      if docstring:
-        node.body.insert(0, docstring)
+            wrapper_node.body = templates.replace(
+                template,
+                body=wrapper_node.body,
+                do_return_var_name=do_return_var_name,
+                function_context=anno.getanno(node, 'function_context_name'),
+                retval_var_name=retval_var_name)
+          else:
+            template = """
+              body
+              return retval_var_name
+            """
+            node.body = templates.replace(
+                template,
+                body=node.body,
+                do_return_var_name=do_return_var_name,
+                retval_var_name=retval_var_name)
 
-    self.state[_Block].exit()
-    self.state[_Function].exit()
     return node
 
 
 def transform(node, ctx, default_to_null_return=True):
-  """Ensure a function has only a single return."""
-  # Note: Technically, these two could be merged into a single walk, but
-  # keeping them separate helps with readability.
-
+  """Ensure a function has only a single return, at the end."""
   node = qual_names.resolve(node)
   node = activity.resolve(node, ctx, None)
 
+  # Note: Technically, these two could be merged into a single walk, but
+  # keeping them separate helps with readability.
   node = ConditionalReturnRewriter(ctx).visit(node)
 
   node = qual_names.resolve(node)
   node = activity.resolve(node, ctx, None)
-
   transformer = ReturnStatementsTransformer(
-      ctx, default_to_null_return=default_to_null_return)
+      ctx, allow_missing_return=default_to_null_return)
   node = transformer.visit(node)
-
   return node
diff --git a/tensorflow/python/autograph/converters/return_statements_test.py b/tensorflow/python/autograph/converters/return_statements_test.py
index df687927638..3f1e6a0bd97 100644
--- a/tensorflow/python/autograph/converters/return_statements_test.py
+++ b/tensorflow/python/autograph/converters/return_statements_test.py
@@ -18,6 +18,7 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
+from tensorflow.python.autograph.converters import functions
 from tensorflow.python.autograph.converters import return_statements
 from tensorflow.python.autograph.core import converter_testing
 from tensorflow.python.framework import ops
@@ -28,7 +29,7 @@ class SingleReturnTest(converter_testing.TestCase):
 
   def assertTransformedEquivalent(self, test_fn, *inputs):
     ns = {'ops': ops}
-    with self.converted(test_fn, return_statements, ns) as result:
+    with self.converted(test_fn, (functions, return_statements), ns) as result:
       self.assertEqual(test_fn(*inputs), result.test_fn(*inputs))
 
   def test_straightline(self):
diff --git a/tensorflow/python/autograph/converters/variables.py b/tensorflow/python/autograph/converters/variables.py
new file mode 100644
index 00000000000..3028a65a69b
--- /dev/null
+++ b/tensorflow/python/autograph/converters/variables.py
@@ -0,0 +1,76 @@
+# Copyright 2020 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Overloads all variable read operations."""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import gast
+
+from tensorflow.python.autograph.core import converter
+from tensorflow.python.autograph.pyct import anno
+from tensorflow.python.autograph.pyct import templates
+
+
+class VariableAccessTransformer(converter.Base):
+  """Rewrites basic symbol reads.
+
+  This transformer rewrites variable reads with a "read" operator which allows
+  tracking activity.
+
+  Example:
+
+  For a basic statement:
+
+      a = b + c
+
+  This is translated to:
+
+      a = ld(b) + ld(c)
+
+  Augmented assignment operations also introduce a `ld` operator:
+
+      a += b
+
+  The assignment target also receives an operator to properly represent the
+  read:
+
+      a = ld(a)
+      a += ld(b)
+  """
+
+  def visit_Name(self, node):
+    # Only the loads which existed in the original code are overloaded.
+    if not anno.hasanno(node, anno.Static.ORIG_DEFINITIONS):
+      return node
+    if isinstance(node.ctx, gast.Load):
+      node = templates.replace_as_expression('ag__.ld(var_)', var_=node)
+    return node
+
+  def visit_AugAssign(self, node):
+    if isinstance(node.target, gast.Name):
+      template = """
+        var_ = ag__.ld(var_)
+        original
+      """
+      node = templates.replace(template, var_=node.target, original=node)
+    else:
+      node = self.generic_visit(node)
+    return node
+
+
+def transform(node, ctx):
+  return VariableAccessTransformer(ctx).visit(node)
diff --git a/tensorflow/python/autograph/converters/variables_test.py b/tensorflow/python/autograph/converters/variables_test.py
new file mode 100644
index 00000000000..556dafbaa8a
--- /dev/null
+++ b/tensorflow/python/autograph/converters/variables_test.py
@@ -0,0 +1,116 @@
+# Copyright 2020 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Tests for variables module."""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import contextlib
+
+from tensorflow.python.autograph.converters import variables
+from tensorflow.python.autograph.core import converter_testing
+from tensorflow.python.platform import test
+
+
+class VariablesTest(converter_testing.TestCase):
+
+  @contextlib.contextmanager
+  def apply_add_one_conversion(self, fn):
+    """Generates code which adds 1 to all variable reads."""
+    with self.converted(fn, variables, {}) as result:
+      result.ag__.__dict__['ld'] = lambda x: x + 1
+      yield result
+
+  def test_read(self):
+
+    def test_fn(l):
+      return l
+
+    with self.apply_add_one_conversion(test_fn) as result:
+      self.assertEqual(result.test_fn(1), 2)
+
+  def test_aug_assign(self):
+
+    def test_fn(l):
+      l *= 10
+      return l
+
+    with self.apply_add_one_conversion(test_fn) as result:
+      self.assertEqual(result.test_fn(1), (1 + 1) * 10 + 1)  # two reads
+
+  def test_attribute(self):
+
+    class TestClass(object):
+
+      def __init__(self):
+        self.v = 1
+
+      def __add__(self, other):
+        self.v += other
+        return self
+
+    def test_fn(l):
+      return l.v
+
+    tc = TestClass()
+    with self.apply_add_one_conversion(test_fn) as result:
+      self.assertEqual(result.test_fn(tc), 2)
+
+  def test_subscript(self):
+
+    class TestClass(object):
+
+      def __init__(self):
+        self.v = 1
+
+      def __add__(self, other):
+        self.v += other
+        return self
+
+      def __getitem__(self, _):
+        return self.v
+
+    def test_fn(l):
+      return l[0]
+
+    tc = TestClass()
+    with self.apply_add_one_conversion(test_fn) as result:
+      self.assertEqual(result.test_fn(tc), 2)
+
+  def test_call(self):
+
+    class TestClass(object):
+
+      def __init__(self):
+        self.v = 1
+
+      def __add__(self, other):
+        self.v += other
+        return self
+
+      def __call__(self):
+        return self.v
+
+    def test_fn(l):
+      return l()
+
+    tc = TestClass()
+    with self.apply_add_one_conversion(test_fn) as result:
+      self.assertEqual(result.test_fn(tc), 2)
+
+
+if __name__ == '__main__':
+  test.main()
diff --git a/tensorflow/python/autograph/core/BUILD b/tensorflow/python/autograph/core/BUILD
index 655dc118a37..4a5c50dac55 100644
--- a/tensorflow/python/autograph/core/BUILD
+++ b/tensorflow/python/autograph/core/BUILD
@@ -30,6 +30,7 @@ py_library(
     visibility = ["//tensorflow:__subpackages__"],
     deps = [
         "//tensorflow/python:framework_ops",
+        "//tensorflow/python/autograph/operators",
         "//tensorflow/python/autograph/pyct",
         "//tensorflow/python/autograph/pyct/static_analysis",
         "//tensorflow/python/autograph/utils",
diff --git a/tensorflow/python/autograph/core/function_wrappers.py b/tensorflow/python/autograph/core/function_wrappers.py
index cc0e7b98de5..d425f8b679d 100644
--- a/tensorflow/python/autograph/core/function_wrappers.py
+++ b/tensorflow/python/autograph/core/function_wrappers.py
@@ -20,12 +20,16 @@ from __future__ import print_function
 
 from tensorflow.python.autograph.core import ag_ctx
 from tensorflow.python.autograph.core import converter
+from tensorflow.python.autograph.operators import variables
 from tensorflow.python.framework import auto_control_deps
 from tensorflow.python.framework import ops
 from tensorflow.python.framework import tensor_util
 from tensorflow.python.util import nest
 
 
+# TODO(mdan): Move this into operators - it represents a function definition.
+
+
 class FunctionScope(object):
   """Context manager that wraps the body of a converted function.
 
@@ -84,8 +88,13 @@ class FunctionScope(object):
     if self.use_auto_deps:
       self.autodeps_scope.__exit__(exc_type, exc_val, exc_tb)
 
-  def mark_return_value(self, value):
+  def ret(self, value, did_return):
     """Marks a value as returned from the function guarded by the scope."""
+    del did_return
+
+    if isinstance(value, variables.UndefinedReturnValue):
+      return None
+
     if self.use_auto_deps:
       self._return_value_marked = True
       if value is None:
diff --git a/tensorflow/python/autograph/core/function_wrappers_test.py b/tensorflow/python/autograph/core/function_wrappers_test.py
index 917a5358633..344ba495570 100644
--- a/tensorflow/python/autograph/core/function_wrappers_test.py
+++ b/tensorflow/python/autograph/core/function_wrappers_test.py
@@ -46,7 +46,7 @@ class FunctionWrappersTest(test.TestCase):
         converter.ConversionOptions(
             optional_features=converter.Feature.AUTO_CONTROL_DEPS)) as scope:
       v.assign(2)
-      op = scope.mark_return_value(constant_op.constant(1))
+      op = scope.ret(constant_op.constant(1), True)
     self.evaluate(op)
     self.assertEqual(self.evaluate(v.read_value()), 2)
 
diff --git a/tensorflow/python/autograph/impl/conversion.py b/tensorflow/python/autograph/impl/conversion.py
index 7a7efe3d43a..eeea0aef896 100644
--- a/tensorflow/python/autograph/impl/conversion.py
+++ b/tensorflow/python/autograph/impl/conversion.py
@@ -36,6 +36,7 @@ from tensorflow.python.autograph.converters import lists
 from tensorflow.python.autograph.converters import logical_expressions
 from tensorflow.python.autograph.converters import return_statements
 from tensorflow.python.autograph.converters import slices
+from tensorflow.python.autograph.converters import variables
 from tensorflow.python.autograph.core import config
 from tensorflow.python.autograph.core import converter
 from tensorflow.python.autograph.core import function_wrappers
@@ -92,6 +93,7 @@ class AutoGraphTranspiler(transpiler.FunctionTranspiler):
     node = control_flow.transform(node, ctx)
     node = conditional_expressions.transform(node, ctx)
     node = logical_expressions.transform(node, ctx)
+    node = variables.transform(node, ctx)
     return node
 
 
diff --git a/tensorflow/python/autograph/operators/BUILD b/tensorflow/python/autograph/operators/BUILD
index 6db9e4f8e3b..3851c7b44ba 100644
--- a/tensorflow/python/autograph/operators/BUILD
+++ b/tensorflow/python/autograph/operators/BUILD
@@ -29,8 +29,7 @@ py_library(
         "logical.py",
         "py_builtins.py",
         "slices.py",
-        "special_values.py",
-        "symbols.py",
+        "variables.py",
     ],
     srcs_version = "PY2AND3",
     visibility = ["//tensorflow:__subpackages__"],
@@ -148,19 +147,8 @@ py_test(
 )
 
 py_test(
-    name = "special_values_test",
-    srcs = ["special_values_test.py"],
-    python_version = "PY3",
-    srcs_version = "PY2AND3",
-    deps = [
-        ":operators",
-        "//tensorflow/python:client_testlib",
-    ],
-)
-
-py_test(
-    name = "symbols_test",
-    srcs = ["symbols_test.py"],
+    name = "variables_test",
+    srcs = ["variables_test.py"],
     python_version = "PY3",
     srcs_version = "PY2AND3",
     deps = [
diff --git a/tensorflow/python/autograph/operators/__init__.py b/tensorflow/python/autograph/operators/__init__.py
index 495b6070aae..f7f9078107c 100644
--- a/tensorflow/python/autograph/operators/__init__.py
+++ b/tensorflow/python/autograph/operators/__init__.py
@@ -60,8 +60,6 @@ from tensorflow.python.autograph.operators.py_builtins import range_
 from tensorflow.python.autograph.operators.slices import get_item
 from tensorflow.python.autograph.operators.slices import GetItemOpts
 from tensorflow.python.autograph.operators.slices import set_item
-from tensorflow.python.autograph.operators.special_values import is_undefined
-from tensorflow.python.autograph.operators.special_values import is_undefined_return
-from tensorflow.python.autograph.operators.special_values import retval
-from tensorflow.python.autograph.operators.special_values import Undefined
-from tensorflow.python.autograph.operators.special_values import UndefinedReturnValue
+from tensorflow.python.autograph.operators.variables import ld
+from tensorflow.python.autograph.operators.variables import Undefined
+from tensorflow.python.autograph.operators.variables import UndefinedReturnValue
diff --git a/tensorflow/python/autograph/operators/control_flow.py b/tensorflow/python/autograph/operators/control_flow.py
index 48b7971ec16..592281b0ce2 100644
--- a/tensorflow/python/autograph/operators/control_flow.py
+++ b/tensorflow/python/autograph/operators/control_flow.py
@@ -65,7 +65,7 @@ import traceback
 import numpy as np
 
 from tensorflow.python.autograph.operators import py_builtins
-from tensorflow.python.autograph.operators import special_values
+from tensorflow.python.autograph.operators import variables
 from tensorflow.python.autograph.utils import ag_logging
 from tensorflow.python.autograph.utils import compat_util
 from tensorflow.python.autograph.utils import misc
@@ -103,13 +103,13 @@ def _verify_loop_init_vars(values, symbol_names):
   for name, value in zip(symbol_names, values):
     if value is None:
       raise ValueError('"{}" may not be None before the loop.'.format(name))
-    if special_values.is_undefined_return(value):
+    if isinstance(value, variables.UndefinedReturnValue):
       # Assumption: the loop will only capture the variable which tracks the
       # return value if the loop contained a return statement.
       # TODO(mdan): This should be checked at the place where return occurs.
       raise ValueError(
           'return statements are not supported within a TensorFlow loop.')
-    if special_values.is_undefined(value):
+    if isinstance(value, variables.Undefined):
       raise ValueError('"{}" must be defined before the loop.'.format(name))
 
 
@@ -495,8 +495,7 @@ def _tf_range_for_stmt(
   iterate = compat_util.BasicRef(start)
 
   def _value_or(name, var, default):
-    if (name == opts['iterate_names']
-        and isinstance(var, special_values.Undefined)):
+    if (name == opts['iterate_names'] and isinstance(var, variables.Undefined)):
       return default
     return var
 
@@ -1019,7 +1018,15 @@ def _wrap_disallow_undefs_from_cond(func, branch_name):
       results_tuple = results
     else:
       results_tuple = results,
-    undefined = tuple(filter(special_values.is_undefined, results_tuple))
+
+    for result in results_tuple:
+      if isinstance(result, variables.UndefinedReturnValue):
+        raise ValueError(
+            'A value must also be returned from the {} branch. If a value is '
+            'returned from one branch of a conditional a value must be '
+            'returned from all branches.'.format(branch_name))
+
+    undefined = [v for v in results_tuple if isinstance(v, variables.Undefined)]
     if undefined:
       raise ValueError(
           'The following symbols must also be initialized in the {} branch: {}.'
@@ -1027,13 +1034,6 @@ def _wrap_disallow_undefs_from_cond(func, branch_name):
           ' statement.'.format(branch_name,
                                tuple(s.symbol_name for s in undefined)))
 
-    for result in results_tuple:
-      if special_values.is_undefined_return(result):
-        raise ValueError(
-            'A value must also be returned from the {} branch. If a value is '
-            'returned from one branch of a conditional a value must be '
-            'returned from all branches.'.format(branch_name))
-
     return results
 
   return wrapper
diff --git a/tensorflow/python/autograph/operators/control_flow_deprecated_py2.py b/tensorflow/python/autograph/operators/control_flow_deprecated_py2.py
index e01a2f206c8..5a900fb19ed 100644
--- a/tensorflow/python/autograph/operators/control_flow_deprecated_py2.py
+++ b/tensorflow/python/autograph/operators/control_flow_deprecated_py2.py
@@ -66,7 +66,7 @@ import functools
 import numpy as np
 
 from tensorflow.python.autograph.operators import py_builtins
-from tensorflow.python.autograph.operators import special_values
+from tensorflow.python.autograph.operators import variables
 from tensorflow.python.autograph.utils import ag_logging
 from tensorflow.python.autograph.utils import misc
 from tensorflow.python.autograph.utils import tensors
@@ -103,13 +103,13 @@ INEFFICIENT_UNROLL_MIN_OPS = 1
 
 def _disallow_undefs_into_loop(*values):
   """Ensures that all values in the state are defined when entering a loop."""
-  undefined = tuple(filter(special_values.is_undefined, values))
+  undefined = [v for v in values if isinstance(v, variables.Undefined)]
   if undefined:
     raise ValueError(
         '{} must be defined before the loop.'.format(
             ','.join(s.symbol_name for s in undefined)))
   for value in values:
-    if special_values.is_undefined_return(value):
+    if isinstance(value, variables.UndefinedReturnValue):
       # Assumption: the loop will only capture the variable which tracks the
       # return value if the loop contained a return statement.
       # TODO(mdan): This should be checked at the place where return occurs.
@@ -1129,7 +1129,7 @@ def _wrap_disallow_undefs_from_cond(func, branch_name):
       results_tuple = results
     else:
       results_tuple = results,
-    undefined = tuple(filter(special_values.is_undefined, results_tuple))
+    undefined = [v for v in results_tuple if isinstance(v, variables.Undefined)]
     if undefined:
       raise ValueError(
           'The following symbols must also be initialized in the {} branch: {}.'
@@ -1138,7 +1138,7 @@ def _wrap_disallow_undefs_from_cond(func, branch_name):
                                tuple(s.symbol_name for s in undefined)))
 
     for result in results_tuple:
-      if special_values.is_undefined_return(result):
+      if isinstance(result, variables.UndefinedReturnValue):
         raise ValueError(
             'A value must also be returned from the {} branch. If a value is '
             'returned from one branch of a conditional a value must be '
diff --git a/tensorflow/python/autograph/operators/control_flow_test.py b/tensorflow/python/autograph/operators/control_flow_test.py
index 5f0a9d09bf3..1c4407904b2 100644
--- a/tensorflow/python/autograph/operators/control_flow_test.py
+++ b/tensorflow/python/autograph/operators/control_flow_test.py
@@ -29,7 +29,7 @@ import numpy as np
 import six
 
 from tensorflow.python.autograph.operators import control_flow
-from tensorflow.python.autograph.operators import special_values
+from tensorflow.python.autograph.operators import variables as variable_operators
 from tensorflow.python.autograph.utils import ag_logging
 from tensorflow.python.data.ops import dataset_ops
 from tensorflow.python.eager import def_function
@@ -546,7 +546,7 @@ class ForLoopTest(test.TestCase):
     with self.assertRaisesRegex(ValueError, '"s" may not be None'):
       self._basic_loop(None, lambda i, s: s)
     with self.assertRaisesRegex(ValueError, '"s" must be defined'):
-      self._basic_loop(special_values.Undefined(''), lambda i, s: s)
+      self._basic_loop(variable_operators.Undefined(''), lambda i, s: s)
 
   def test_tensor_none_output(self):
     with self.assertRaisesRegex(ValueError, '"s" is None at the end'):
@@ -785,7 +785,7 @@ class WhileLoopTest(test.TestCase):
     with self.assertRaisesRegex(ValueError, '"s" may not be None'):
       self._basic_loop(None, lambda i, s: s)
     with self.assertRaisesRegex(ValueError, '"s" must be defined'):
-      self._basic_loop(special_values.Undefined(''), lambda i, s: s)
+      self._basic_loop(variable_operators.Undefined(''), lambda i, s: s)
 
   def test_tensor_none_output(self):
     with self.assertRaisesRegex(ValueError, '"s" is None at the end'):
@@ -887,10 +887,10 @@ class IfStmtTest(test.TestCase):
   def test_tensor_undefined_output(self):
     with self.assertRaisesRegex(
         ValueError, "must also be initialized in the if.*'s'"):
-      self._basic_cond(lambda: special_values.Undefined('s'), lambda: 1)
+      self._basic_cond(lambda: variable_operators.Undefined('s'), lambda: 1)
     with self.assertRaisesRegex(
         ValueError, "must also be initialized in the else.*'s'"):
-      self._basic_cond(lambda: 1, lambda: special_values.Undefined('s'))
+      self._basic_cond(lambda: 1, lambda: variable_operators.Undefined('s'))
 
   def test_tensor_dtype_change(self):
     with self.assertRaisesRegex(TypeError, '"s" has dtype int32.*but.*float32'):
diff --git a/tensorflow/python/autograph/operators/symbols.py b/tensorflow/python/autograph/operators/symbols.py
deleted file mode 100644
index 0dd7e0a5956..00000000000
--- a/tensorflow/python/autograph/operators/symbols.py
+++ /dev/null
@@ -1,115 +0,0 @@
-# Copyright 2017 The TensorFlow Authors. All Rights Reserved.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-# ==============================================================================
-"""Abstract representation of composite symbols that can be used in staging code.
-
-This provides a way to checkpoint the values of symbols that may be undefined
-entering staged control flow. This checkpointing is necessary to prevent some
-unintended side-effects. For example checkpointing prevents side-effects in one
-branch of a conditional from leaking into another.
-"""
-
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-
-from tensorflow.python.autograph.operators import special_values
-
-
-is_undefined = special_values.is_undefined
-Undefined = special_values.Undefined
-
-
-class Symbol(object):
-  """Representation of a simple or composite Python symbol.
-
-  Subclasses should implement `maybe_compute_value(self)` that returns the value
-  corresponding to the symbol or Undefined if no such value exists.
-  """
-
-  def __init__(self, name):
-    self.name = name
-
-
-class ValueSymbol(Symbol):
-  """Representation of a simple Python symbol with a concrete value.
-
-  This includes variables and literals. Since we are reifying undefined symbols
-  `Undefined` is also a valid value.
-  """
-
-  def __init__(self, name, value):
-    super(ValueSymbol, self).__init__(name)
-    self.value = value
-
-  def maybe_compute_value(self):
-    return self.value
-
-
-class AttributeAccessSymbol(Symbol):
-  """Representation of Python attribute access e.g. `a.b`."""
-
-  def __init__(self, parent_symbol, attr_name):
-    super(AttributeAccessSymbol, self).__init__(
-        parent_symbol.name + '.' + attr_name)
-    self.attr_name = attr_name
-    self.parent_symbol = parent_symbol
-
-  def maybe_compute_value(self):
-    """Compute the value corresponding to the attribute access or `Undefined`.
-
-    This will be `Undefined` if no such value exists either because there is no
-    such attribute or if the base is itself undefined.
-
-    Returns:
-      value corresponding to the attribute access or `Undefined`
-    """
-    parent_value = self.parent_symbol.maybe_compute_value()
-    if (is_undefined(parent_value) or
-        getattr(parent_value, self.attr_name, None) is None):
-      return Undefined(self.name)
-
-    return parent_value.__getattribute__(self.attr_name)
-
-
-class SubscriptSymbol(Symbol):
-  """Representation of Python subscript access e.g. `a[b]`."""
-
-  def __init__(self, parent_symbol, index_symbol):
-    super(SubscriptSymbol, self).__init__(
-        parent_symbol.name + '[' + index_symbol.name + ']')
-    self.index_symbol = index_symbol
-    self.parent_symbol = parent_symbol
-
-  def maybe_compute_value(self):
-    """Compute the value corresponding to the subscript access or `Undefined`.
-
-    This will be `Undefined` if no such value exists either because there is no
-    element corresponding to the given subscript or if the base itself is
-    not defined.
-
-    Returns:
-      value corresponding to the subscript access or `Undefined`
-    """
-    parent_value = self.parent_symbol.maybe_compute_value()
-    index_value = self.index_symbol.maybe_compute_value()
-    if is_undefined(parent_value) or is_undefined(index_value):
-      return Undefined(self.name)
-
-    try:
-      return parent_value[index_value]
-    except (IndexError, KeyError, TypeError):
-      # Reify the lack of an object for the given index/key
-      # This allows us to define them later without regret
-      return Undefined(self.name)
diff --git a/tensorflow/python/autograph/operators/symbols_test.py b/tensorflow/python/autograph/operators/symbols_test.py
deleted file mode 100644
index 3acb16273bd..00000000000
--- a/tensorflow/python/autograph/operators/symbols_test.py
+++ /dev/null
@@ -1,230 +0,0 @@
-# Copyright 2019 The TensorFlow Authors. All Rights Reserved.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-# ==============================================================================
-"""Tests for special symbol handling."""
-
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-
-from tensorflow.python.autograph.operators import special_values
-from tensorflow.python.autograph.operators import symbols
-from tensorflow.python.platform import test
-
-Undefined = special_values.Undefined
-AttributeAccessSymbol = symbols.AttributeAccessSymbol
-SubscriptSymbol = symbols.SubscriptSymbol
-ValueSymbol = symbols.ValueSymbol
-
-
-class SymbolsTest(test.TestCase):
-
-  def test_value_symbol_returns_value(self):
-    a = 42
-    a_symbol = ValueSymbol('a', a)
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(a_symbol.name, 'a')
-
-  def test_attribute_access_missing_attribute(self):
-    class Foo(object):
-      pass
-    a = Foo()
-
-    a_symbol = ValueSymbol('a', a)
-    a_b_symbol = AttributeAccessSymbol(a_symbol, 'b')
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertIsInstance(a_b_symbol.maybe_compute_value(), Undefined)
-    self.assertEqual(a_b_symbol.maybe_compute_value().symbol_name, 'a.b')
-
-  def test_attribute_access_undefined_target(self):
-    a = Undefined('a')
-    a_symbol = ValueSymbol('a', a)
-    a_b_symbol = AttributeAccessSymbol(a_symbol, 'b')
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertIsInstance(a_b_symbol.maybe_compute_value(), Undefined)
-    self.assertEqual(a_b_symbol.maybe_compute_value().symbol_name, 'a.b')
-
-  def test_attribute_access_basic(self):
-    class Foo(object):
-
-      def __init__(self):
-        self.b = 'this is an attribute'
-
-    a = Foo()
-    a_symbol = ValueSymbol('a', a)
-    a_b_symbol = AttributeAccessSymbol(a_symbol, 'b')
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(a_b_symbol.maybe_compute_value(), a.b)
-
-  def test_item_access_undefined_index(self):
-    class Foo(object):
-
-      def __getitem__(self, key):
-        return 'this is an item'
-
-    a = Foo()
-    b = Undefined('b')
-    a_symbol = ValueSymbol('a', a)
-    b_symbol = ValueSymbol('b', b)
-    a_b_symbol = SubscriptSymbol(a_symbol, b_symbol)
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(b_symbol.maybe_compute_value(), b)
-    self.assertIsInstance(a_b_symbol.maybe_compute_value(), Undefined)
-    self.assertEqual(a_b_symbol.maybe_compute_value().symbol_name, 'a[b]')
-
-  def test_item_access_no_getitem(self):
-    class Foo(object):
-      pass
-
-    a = Foo()
-    b = 42
-    a_symbol = ValueSymbol('a', a)
-    b_symbol = ValueSymbol('b', b)
-    a_b_symbol = SubscriptSymbol(a_symbol, b_symbol)
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(b_symbol.maybe_compute_value(), b)
-    self.assertIsInstance(a_b_symbol.maybe_compute_value(), Undefined)
-    self.assertEqual(a_b_symbol.maybe_compute_value().symbol_name, 'a[b]')
-
-  def test_item_access_undefined_root(self):
-    a = Undefined('a')
-    b = 42
-    a_symbol = ValueSymbol('a', a)
-    b_symbol = ValueSymbol('b', b)
-    a_b_symbol = SubscriptSymbol(a_symbol, b_symbol)
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(b_symbol.maybe_compute_value(), b)
-    self.assertIsInstance(a_b_symbol.maybe_compute_value(), Undefined)
-    self.assertEqual(a_b_symbol.maybe_compute_value().symbol_name, 'a[b]')
-
-  def test_item_access_basic(self):
-    class Foo(object):
-
-      def __getitem__(self, key):
-        return 'this is an item'
-
-    a = Foo()
-    b = 42
-    a_symbol = ValueSymbol('a', a)
-    b_symbol = ValueSymbol('b', b)
-    a_b_symbol = SubscriptSymbol(a_symbol, b_symbol)
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(b_symbol.maybe_compute_value(), b)
-    self.assertEqual(a_b_symbol.maybe_compute_value(), a[b])
-
-  def test_item_access_after_attribute_access(self):
-    class Foo(object):
-
-      def __getitem__(self, key):
-        return 'this is an item'
-
-    class Bar(object):
-
-      def __init__(self):
-        self.b = Foo()
-
-    a = Bar()
-    c = 42
-    a_symbol = ValueSymbol('a', a)
-    c_symbol = ValueSymbol('c', c)
-    a_b_symbol = AttributeAccessSymbol(a_symbol, 'b')
-    a_b_c_symbol = SubscriptSymbol(a_b_symbol, c_symbol)
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(c_symbol.maybe_compute_value(), c)
-    self.assertEqual(a_b_symbol.maybe_compute_value(), a.b)
-    self.assertEqual(a_b_c_symbol.maybe_compute_value(), a.b[c])
-
-  def test_attribute_access_after_item_access(self):
-    class Bar(object):
-
-      def __init__(self):
-        self.c = object()
-
-    item = Bar()
-
-    class Foo(object):
-
-      def __getitem__(self, key):
-        return item
-
-    a = Foo()
-    b = 42
-    a_symbol = ValueSymbol('a', a)
-    b_symbol = ValueSymbol('b', b)
-    a_b_symbol = SubscriptSymbol(a_symbol, b_symbol)
-    a_b_c_symbol = AttributeAccessSymbol(a_b_symbol, 'c')
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(b_symbol.maybe_compute_value(), b)
-    self.assertEqual(a_b_symbol.maybe_compute_value(), a[b])
-    self.assertEqual(a_b_c_symbol.maybe_compute_value(), a[b].c)
-
-  def test_item_access_after_item_access(self):
-    class Bar(object):
-
-      def __getitem__(self, key):
-        return 'this is an item'
-
-    item = Bar()
-
-    class Foo(object):
-
-      def __getitem__(self, key):
-        return item
-
-    a = Foo()
-    b = 42
-    c = 43
-    a_symbol = ValueSymbol('a', a)
-    b_symbol = ValueSymbol('b', b)
-    c_symbol = ValueSymbol('b', c)
-    a_b_symbol = SubscriptSymbol(a_symbol, b_symbol)
-    a_b_c_symbol = SubscriptSymbol(a_b_symbol, c_symbol)
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(b_symbol.maybe_compute_value(), b)
-    self.assertEqual(a_b_symbol.maybe_compute_value(), a[b])
-    self.assertEqual(a_b_c_symbol.maybe_compute_value(), a[b][c])
-
-  def test_attribute_access_after_attribute_access(self):
-    class Bar(object):
-
-      def __init__(self):
-        self.c = object()
-
-    class Foo(object):
-
-      def __init__(self):
-        self.b = Bar()
-
-    a = Foo()
-    a_symbol = ValueSymbol('a', a)
-    a_b_symbol = AttributeAccessSymbol(a_symbol, 'b')
-    a_b_c_symbol = AttributeAccessSymbol(a_b_symbol, 'c')
-
-    self.assertEqual(a_symbol.maybe_compute_value(), a)
-    self.assertEqual(a_b_symbol.maybe_compute_value(), a.b)
-    self.assertEqual(a_b_c_symbol.maybe_compute_value(), a.b.c)
-
-
-if __name__ == '__main__':
-  test.main()
diff --git a/tensorflow/python/autograph/operators/special_values.py b/tensorflow/python/autograph/operators/variables.py
similarity index 72%
rename from tensorflow/python/autograph/operators/special_values.py
rename to tensorflow/python/autograph/operators/variables.py
index c172cce23f1..150f64e1758 100644
--- a/tensorflow/python/autograph/operators/special_values.py
+++ b/tensorflow/python/autograph/operators/variables.py
@@ -19,6 +19,13 @@ from __future__ import division
 from __future__ import print_function
 
 
+def ld(v):
+  """Load variable operator."""
+  if isinstance(v, Undefined):
+    return v.read()
+  return v
+
+
 class Undefined(object):
   """Represents an undefined symbol in Python.
 
@@ -51,6 +58,10 @@ class Undefined(object):
   def __init__(self, symbol_name):
     self.symbol_name = symbol_name
 
+  def read(self):
+    raise UnboundLocalError("'{}' is used before assignment".format(
+        self.symbol_name))
+
   def __repr__(self):
     return self.symbol_name
 
@@ -66,34 +77,7 @@ class Undefined(object):
     return self
 
 
-def is_undefined(value):
-  """Checks whether Autograph has determined that a given value is undefined.
-
-  This only works in places where Autograph reifies undefined symbols. Note that
-  if this function is passed a truly undefined symbol the call-site will raise
-  NameError.
-
-  Args:
-    value: value to test for undefinedness
-  Returns:
-    Boolean, whether the input value is undefined.
-  """
-  return isinstance(value, Undefined)
-
-
 # TODO(mdan): Refactor as a RetVal object, aggregating the value and do_return.
 class UndefinedReturnValue(object):
-  """Represents a default return value from a function (None in Python)."""
+  """Represents a return value that is undefined."""
   pass
-
-
-def retval(value):
-  """Returns the actual value that a return statement should produce."""
-  if isinstance(value, UndefinedReturnValue):
-    return None
-  return value
-
-
-def is_undefined_return(value):
-  """Checks whether `value` is the default return value."""
-  return isinstance(value, UndefinedReturnValue)
diff --git a/tensorflow/python/autograph/operators/special_values_test.py b/tensorflow/python/autograph/operators/variables_test.py
similarity index 58%
rename from tensorflow/python/autograph/operators/special_values_test.py
rename to tensorflow/python/autograph/operators/variables_test.py
index 1742cc4277d..168e6172232 100644
--- a/tensorflow/python/autograph/operators/special_values_test.py
+++ b/tensorflow/python/autograph/operators/variables_test.py
@@ -18,28 +18,38 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
-from tensorflow.python.autograph.operators import special_values
+from tensorflow.python.autograph.operators import variables
 from tensorflow.python.platform import test
 
 
 class SpecialValuesTest(test.TestCase):
 
   def test_undefined(self):
-    undefined_symbol = special_values.Undefined('name')
-    self.assertEqual(undefined_symbol.symbol_name, 'name')
+    undefined_symbol = variables.Undefined('name')
+    undefined_symbol2 = variables.Undefined('name')
 
-    undefined_symbol2 = special_values.Undefined('name')
+    self.assertEqual(undefined_symbol.symbol_name, 'name')
+    self.assertEqual(undefined_symbol2.symbol_name, 'name')
     self.assertNotEqual(undefined_symbol, undefined_symbol2)
 
-    self.assertTrue(special_values.is_undefined(undefined_symbol))
-    self.assertTrue(special_values.is_undefined(undefined_symbol2))
-
   def test_undefined_operations(self):
-    undefined_symbol = special_values.Undefined('name')
+    undefined_symbol = variables.Undefined('name')
+
+    self.assertIsInstance(undefined_symbol.foo, variables.Undefined)
+    self.assertIsInstance(undefined_symbol[0], variables.Undefined)
+    self.assertNotIsInstance(undefined_symbol.__class__, variables.Undefined)
+
+  def test_read(self):
+    self.assertEqual(variables.ld(1), 1)
+    o = object()
+    self.assertEqual(variables.ld(o), o)
+
+    self.assertIsNone(variables.ld(None))
+
+  def test_read_undefined(self):
+    with self.assertRaisesRegex(UnboundLocalError, 'used before assignment'):
+      variables.ld(variables.Undefined('a'))
 
-    self.assertTrue(special_values.is_undefined(undefined_symbol.foo))
-    self.assertTrue(special_values.is_undefined(undefined_symbol[0]))
-    self.assertFalse(special_values.is_undefined(undefined_symbol.__class__))
 
 if __name__ == '__main__':
   test.main()
diff --git a/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_py3_test.py b/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_py3_test.py
index 7333ec0c872..ba27280f729 100644
--- a/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_py3_test.py
+++ b/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_py3_test.py
@@ -78,6 +78,18 @@ class ReachingDefinitionsAnalyzerTest(
 
     self.assertSameDef(local_body[1].test, local_body[2].value.elts[0])
 
+    # Note: the function name is is visible inside the function body. But it's
+    # a closure variable, not a local.
+    #
+    # Example:
+    #
+    #   >>> def f():
+    #   ...  print(f)
+    #   >>> g = f
+    #   >>> f = 'something else'
+    #   >>> g()
+    #   something else
+    #
     self.assertHasDefinedIn(local_body[1], ('a', 'b'))
 
 
diff --git a/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_test.py b/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_test.py
index c4e7cbd4d17..64b00fcbeba 100644
--- a/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_test.py
+++ b/tensorflow/python/autograph/pyct/static_analysis/reaching_definitions_test.py
@@ -255,6 +255,9 @@ class ReachingDefinitionsAnalyzerTest(ReachingDefinitionsAnalyzerTestBase):
 
     inner_fn_body = fn_body[1].body[1].body
     def_of_a_in_foo = inner_fn_body[0].value
+    # Even though `a` is visible in the inner functio above, the late binding
+    # makes it impossible to assume that the same value will be visible at
+    # call time.
     self.assertHasDefs(def_of_a_in_foo, 0)
 
   def test_nested_functions_isolation(self):