Correctly resolve None items inside ListPlaceholder

tfx/dsl/compiler/placeholder_utils.py

@@ -433,9 +433,10 @@ def _resolve_list_concat_operator(
     """Evaluates the list concat operator."""
     result = []
     for sub_expression in op.expressions:
-      value = self.resolve(sub_expression, pool)
-      if value is None:
-        raise NullDereferenceError(sub_expression)
+      try:
+        value = self.resolve(sub_expression, pool)
+      except NullDereferenceError:
+        value = None
       result.append(value)
     return result
 

tfx/dsl/compiler/placeholder_utils_test.py

@@ -665,6 +665,43 @@ def testListConcat(self):
         placeholder_utils.resolve_placeholder_expression(
             pb, self._resolution_context), expected_result)
 
+  def testListConcatWithAbsentElement(self):
+    # When an exec prop has type Union[T, None] and the user passes None, it is
+    # actually completely absent from the exec_properties dict in
+    # ExecutionInvocation. See also b/172001324 and the corresponding todo in
+    # placeholder_utils.py.
+    placeholder_expression = """
+      operator {
+        list_concat_op {
+          expressions {
+            value {
+              string_value: "random_before"
+            }
+          }
+          expressions {
+            placeholder {
+              type: EXEC_PROPERTY
+              key: "doesnotexist"
+            }
+          }
+          expressions {
+            value {
+              string_value: "random_after"
+            }
+          }
+        }
+      }
+    """
+    pb = text_format.Parse(
+        placeholder_expression, placeholder_pb2.PlaceholderExpression()
+    )
+    self.assertEqual(
+        placeholder_utils.resolve_placeholder_expression(
+            pb, self._resolution_context
+        ),
+        ["random_before", None, "random_after"],
+    )
+
   def testListConcatAndSerialize(self):
     placeholder_expression = """
       operator {

tfx/dsl/placeholder/placeholder_base.py

@@ -354,8 +354,7 @@ def serialize_list(
     """Serializes list-value placeholder to JSON or comma-separated string.
 
     Only supports primitive type list element (a.k.a bool, int, float or str) at
-    the
-    moment; throws runtime error otherwise.
+    the moment; throws runtime error otherwise.
 
     Args:
        serialization_format: The format of how the proto is serialized.

tfx/dsl/placeholder/proto_placeholder_test.py

@@ -220,7 +220,8 @@ def test_NonePlaceholderIntoOptionalField(self):
   def test_NoneExecPropIntoOptionalField(self):
     # When an exec prop has type Union[T, None] and the user passes None, it is
     # actually completely absent from the exec_properties dict in
-    # ExecutionInvocation.
+    # ExecutionInvocation. See also b/172001324 and the corresponding todo in
+    # placeholder_utils.py.
     actual = resolve(
         _UpdateOptions(reload_policy=ph.exec_property('reload_policy')),
         exec_properties={},  # Intentionally empty.
@@ -385,6 +386,33 @@ def test_RepeatedFieldFalsyItem(self):
         parse_text_proto(actual),
     )
 
+  def test_RepeatedFieldNoneItem(self):
+    actual = resolve(
+        ph.make_proto(
+            execution_invocation_pb2.ExecutionInvocation(
+                pipeline_node=pipeline_pb2.PipelineNode()
+            ),
+            pipeline_node=ph.make_proto(
+                pipeline_pb2.PipelineNode(),
+                upstream_nodes=[
+                    'foo',
+                    ph.exec_property('reload_policy'),  # Will be None.
+                    'bar',
+                ],
+            ),
+        ),
+        exec_properties={},  # Intentionally empty.
+    )
+    self.assertProtoEquals(
+        """
+        pipeline_node {
+          upstream_nodes: "foo"
+          upstream_nodes: "bar"
+        }
+        """,
+        parse_text_proto(actual),
+    )
+
   def test_NoneIntoRepeatedField(self):
     actual = resolve(
         ph.make_proto(pipeline_pb2.PipelineNode(), upstream_nodes=None)