Better error messaging for chain-type pipeline (#33210)

sdks/python/apache_beam/yaml/yaml_transform.py

@@ -576,8 +576,8 @@ def is_not_output_of_last_transform(new_transforms, value):
         pass
       else:
         raise ValueError(
-            f'Transform {identify_object(transform)} is part of a chain, '
-            'must have implicit inputs and outputs.')
+            f'Transform {identify_object(transform)} is part of a chain. '
+            'Cannot define explicit inputs on chain pipeline')
     if ix == 0:
       if is_explicitly_empty(transform.get('input', None)):
         pass

sdks/python/apache_beam/yaml/yaml_transform_unit_test.py

@@ -325,6 +325,23 @@ def test_chain_as_composite_with_input(self):
     self.assertEqual(
         chain_as_composite(spec)['transforms'][0]['input'], {"input": "input"})
 
+  def test_chain_as_composite_with_transform_input(self):
+    spec = '''
+        type: chain
+        transforms:
+        - type: Create
+          config:
+            elements: [0,1,2]
+        - type: LogForTesting
+          input: Create
+      '''
+    spec = yaml.load(spec, Loader=SafeLineLoader)
+    with self.assertRaisesRegex(
+        ValueError,
+        r"Transform .* is part of a chain. "
+        r"Cannot define explicit inputs on chain pipeline"):
+      chain_as_composite(spec)
+
   def test_normalize_source_sink(self):
     spec = '''
         source: