fix(sdk): Prevents dsl.ParallelFor over single parameter from compiling.

sdk/RELEASE.md

@@ -7,6 +7,7 @@
 ## Deprecations
 
 ## Bug fixes and other changes
+* Throw compilation error when trying to iterate over a single parameter with ParallelFor [\#10494](https://github.com/kubeflow/pipelines/pull/10494)
 
 ## Documentation updates
 

sdk/python/kfp/compiler/compiler_test.py

@@ -825,6 +825,19 @@ def my_pipeline(text: bool):
         with self.assertRaises(KeyError):
             for_loop_4['iteratorPolicy']
 
+    def test_cannot_compile_parallel_for_with_single_param(self):
+
+        with self.assertRaisesRegex(
+                ValueError,
+                r'Cannot iterate over a single parameter using `dsl\.ParallelFor`\. Expected a list of parameters as argument to `items`\.'
+        ):
+
+            @dsl.pipeline
+            def my_pipeline():
+                single_param_task = print_and_return(text='string')
+                with dsl.ParallelFor(items=single_param_task.output) as item:
+                    print_and_return(text=item)
+
     def test_pipeline_in_pipeline(self):
 
         @dsl.pipeline(name='graph-component')

sdk/python/kfp/dsl/for_loop.py

@@ -17,6 +17,8 @@
 from typing import Any, Dict, List, Optional, Union
 
 from kfp.dsl import pipeline_channel
+from kfp.dsl.types import type_annotations
+from kfp.dsl.types import type_utils
 
 ItemList = List[Union[int, float, str, Dict[str, Any]]]
 
@@ -124,7 +126,7 @@ def __init__(
                 Python variable name.
             name_code: A unique code used to identify these loop arguments.
                 Should match the code for the ParallelFor ops_group which created
-                these LoopArguments. This prevents parameter name collisions.
+                these LoopParameterArguments. This prevents parameter name collisions.
             name_override: The override name for PipelineParameterChannel.
             **kwargs: Any other keyword arguments passed down to PipelineParameterChannel.
         """
@@ -166,7 +168,7 @@ def __init__(
 
     def __getattr__(self, name: str):
         # this is being overridden so that we can access subvariables of the
-        # LoopArgument (i.e.: item.a) without knowing the subvariable names ahead
+        # LoopParameterArgument (i.e.: item.a) without knowing the subvariable names ahead
         # of time.
 
         return self._referenced_subvars.setdefault(
@@ -188,6 +190,17 @@ def from_pipeline_channel(
         compilation progress in cases of unknown or missing type
         information.
         """
+        # if channel is a LoopArgumentVariable, current system cannot check if
+        # nested items are lists.
+        if not isinstance(channel, LoopArgumentVariable):
+            type_name = type_annotations.get_short_type_name(
+                channel.channel_type)
+            parameter_type = type_utils.PARAMETER_TYPES_MAPPING[
+                type_name.lower()]
+            if parameter_type != type_utils.LIST:
+                raise ValueError(
+                    'Cannot iterate over a single parameter using `dsl.ParallelFor`. Expected a list of parameters as argument to `items`.'
+                )
         return LoopParameterArgument(
             items=channel,
             name_override=channel.name + '-' + LOOP_ITEM_NAME_BASE,
@@ -297,7 +310,7 @@ class LoopArgumentVariable(pipeline_channel.PipelineParameterChannel):
     Then there's one LoopArgumentVariable for 'a' and another for 'b'.
 
     Attributes:
-        loop_argument: The original LoopArgument object this subvariable is
+        loop_argument: The original LoopParameterArgument object this subvariable is
           attached to.
         subvar_name: The subvariable name.
     """
@@ -327,7 +340,7 @@ def __init__(
 
         self.subvar_name = subvar_name
         self.loop_argument = loop_argument
-        # Handle potential channel_type extraction errors from LoopArgument by defaulting to 'String'. This maintains compilation progress.
+        # Handle potential channel_type extraction errors from LoopParameterArgument by defaulting to 'String'. This maintains compilation progress.
         super().__init__(
             name=self._get_name_override(
                 loop_arg_name=loop_argument.name,

sdk/python/kfp/dsl/for_loop_test.py

@@ -144,6 +144,24 @@ def test_loop_parameter_argument_from_pipeline_channel(
         self.assertEqual(loop_argument.items_or_pipeline_channel, channel)
         self.assertEqual(str(loop_argument), expected_serialization_value)
 
+    @parameterized.parameters(
+        {
+            'channel':
+                pipeline_channel.PipelineParameterChannel(
+                    name='param1',
+                    channel_type='String',
+                    task_name='task1',
+                ),
+        },)
+    def test_loop_parameter_argument_from_single_pipeline_channel_raises_error(
+            self, channel):
+        with self.assertRaisesRegex(
+                ValueError,
+                r'Cannot iterate over a single parameter using `dsl\.ParallelFor`\. Expected a list of parameters as argument to `items`\.'
+        ):
+            loop_argument = for_loop.LoopParameterArgument.from_pipeline_channel(
+                channel)
+
     @parameterized.parameters(
         {
             'channel':

sdk/python/kfp/dsl/types/type_utils_test.py

@@ -720,7 +720,7 @@ class TestTypeChecking(parameterized.TestCase):
                     loop_argument=for_loop.LoopParameterArgument
                     .from_pipeline_channel(
                         pipeline_channel.create_pipeline_channel(
-                            'Output-loop-item', 'String',
+                            'Output-loop-item', 'List[str]',
                             'list-dict-without-type-maker-5')),
                     subvar_name='a'),
             'parameter_input_spec':
@@ -732,7 +732,7 @@ class TestTypeChecking(parameterized.TestCase):
             'argument_value':
                 for_loop.LoopParameterArgument.from_pipeline_channel(
                     pipeline_channel.create_pipeline_channel(
-                        'Output-loop-item', 'String',
+                        'Output-loop-item', 'List[int]',
                         'list-dict-without-type-maker-5')),
             'parameter_input_spec':
                 structures.InputSpec('Integer'),