Allow undefined symbols to be queried for attributes and slices. An upcoming change will create situations when this can happen before the verification for undefined symbols that currently precludes such operations.

PiperOrigin-RevId: 288336945
Change-Id: I5bb1e02a9b3dacb557749ada4aaffe402c17cd82

tensorflow/python/autograph/operators/special_values.py

@@ -46,10 +46,25 @@ class Undefined(object):
     symbol_name: Text, identifier for the undefined symbol
   """
 
+  __slots__ = ('symbol_name',)
+
   def __init__(self, symbol_name):
-    # TODO(aqj) Possibly remove this after Symbols are fully integrated.
     self.symbol_name = symbol_name
 
+  def __repr__(self):
+    return self.symbol_name
+
+  def __getattribute__(self, name):
+    try:
+      # If it's an existing attribute, return it.
+      return object.__getattribute__(self, name)
+    except AttributeError:
+      # Otherwise return Undefined.
+      return self
+
+  def __getitem__(self, i):
+    return self
+
 
 def is_undefined(value):
   """Checks whether Autograph has determined that a given value is undefined.

tensorflow/python/autograph/operators/special_values_test.py

@@ -34,5 +34,12 @@ class SpecialValuesTest(test.TestCase):
     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')
+
+    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()