fix: [Bug] Pipeline parameters (ParameterInteger, ParameterString) fail in ModelTrain (5504)

sagemaker-train/src/sagemaker/train/utils.py

@@ -192,7 +192,16 @@ def safe_serialize(data):
     try:
         return json.dumps(data)
     except TypeError:
-        return str(data)
+        try:
+            return str(data)
+        except TypeError:
+            # PipelineVariable.__str__ raises TypeError by design.
+            # If the isinstance check above didn't catch it (e.g. import
+            # path mismatch), fall back to returning the object directly
+            # when it looks like a PipelineVariable (has an ``expr`` property).
+            if hasattr(data, "expr"):
+                return data
+            raise
 
 
 def _run_clone_command_silent(repo_url, dest_dir):

sagemaker-train/tests/unit/train/test_model_trainer_pipeline_variable.py

@@ -26,13 +26,14 @@
 
 from sagemaker.core.helper.session_helper import Session
 from sagemaker.core.helper.pipeline_variable import PipelineVariable, StrPipeVar
-from sagemaker.core.workflow.parameters import ParameterString
+from sagemaker.core.workflow.parameters import ParameterString, ParameterInteger, ParameterFloat
 from sagemaker.train.model_trainer import ModelTrainer, Mode
 from sagemaker.train.configs import (
     Compute,
     StoppingCondition,
     OutputDataConfig,
 )
+from sagemaker.core.workflow.pipeline_context import PipelineSession
 from sagemaker.train.defaults import DEFAULT_INSTANCE_TYPE
 
 
@@ -176,3 +177,61 @@ def test_training_image_rejects_invalid_type(self):
                 stopping_condition=DEFAULT_STOPPING,
                 output_data_config=DEFAULT_OUTPUT,
             )
+
+
+class TestModelTrainerHyperparametersPipelineVariable:
+    """Test that PipelineVariable objects in hyperparameters survive safe_serialize."""
+
+    def test_hyperparameters_with_pipeline_variable_integer(self):
+        """ParameterInteger in hyperparameters should be passed through as-is."""
+        max_depth = ParameterInteger(name="MaxDepth", default_value=5)
+        trainer = ModelTrainer(
+            training_image=DEFAULT_IMAGE,
+            role=DEFAULT_ROLE,
+            compute=DEFAULT_COMPUTE,
+            stopping_condition=DEFAULT_STOPPING,
+            output_data_config=DEFAULT_OUTPUT,
+            hyperparameters={"max_depth": max_depth},
+        )
+        # safe_serialize should return the PipelineVariable object directly
+        from sagemaker.train.utils import safe_serialize
+        result = safe_serialize(max_depth)
+        assert result is max_depth
+
+    def test_hyperparameters_with_pipeline_variable_string(self):
+        """ParameterString in hyperparameters should be passed through as-is."""
+        optimizer = ParameterString(name="Optimizer", default_value="sgd")
+        trainer = ModelTrainer(
+            training_image=DEFAULT_IMAGE,
+            role=DEFAULT_ROLE,
+            compute=DEFAULT_COMPUTE,
+            stopping_condition=DEFAULT_STOPPING,
+            output_data_config=DEFAULT_OUTPUT,
+            hyperparameters={"optimizer": optimizer},
+        )
+        from sagemaker.train.utils import safe_serialize
+        result = safe_serialize(optimizer)
+        assert result is optimizer
+
+    def test_hyperparameters_with_mixed_pipeline_and_regular_values(self):
+        """Mixed PipelineVariable and regular values should both serialize correctly."""
+        max_depth = ParameterInteger(name="MaxDepth", default_value=5)
+        trainer = ModelTrainer(
+            training_image=DEFAULT_IMAGE,
+            role=DEFAULT_ROLE,
+            compute=DEFAULT_COMPUTE,
+            stopping_condition=DEFAULT_STOPPING,
+            output_data_config=DEFAULT_OUTPUT,
+            hyperparameters={
+                "max_depth": max_depth,
+                "eta": 0.1,
+                "objective": "binary:logistic",
+            },
+        )
+        from sagemaker.train.utils import safe_serialize
+        # PipelineVariable should be returned as-is
+        assert safe_serialize(max_depth) is max_depth
+        # Float should be JSON-serialized
+        assert safe_serialize(0.1) == "0.1"
+        # String should be returned as-is
+        assert safe_serialize("binary:logistic") == "binary:logistic"

sagemaker-train/tests/unit/train/test_utils.py

@@ -0,0 +1,90 @@
+# Copyright Amazon.com, Inc. or its affiliates. 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. A copy of
+# the License is located at
+#
+#     http://aws.amazon.com/apache2.0/
+#
+# or in the "license" file accompanying this file. This file 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.
+"""Unit tests for sagemaker.train.utils – specifically safe_serialize."""
+from __future__ import absolute_import
+
+import json
+
+from sagemaker.train.utils import safe_serialize
+from sagemaker.core.workflow.parameters import (
+    ParameterInteger,
+    ParameterString,
+    ParameterFloat,
+)
+
+
+# ---------------------------------------------------------------------------
+# PipelineVariable inputs – should be returned as-is (identity)
+# ---------------------------------------------------------------------------
+
+def test_safe_serialize_with_pipeline_variable_integer_returns_object_directly():
+    param = ParameterInteger(name="MaxDepth", default_value=5)
+    result = safe_serialize(param)
+    assert result is param
+
+
+def test_safe_serialize_with_pipeline_variable_string_returns_object_directly():
+    param = ParameterString(name="Optimizer", default_value="sgd")
+    result = safe_serialize(param)
+    assert result is param
+
+
+def test_safe_serialize_with_pipeline_variable_float_returns_object_directly():
+    param = ParameterFloat(name="LearningRate", default_value=0.01)
+    result = safe_serialize(param)
+    assert result is param
+
+
+# ---------------------------------------------------------------------------
+# Regular / primitive inputs
+# ---------------------------------------------------------------------------
+
+def test_safe_serialize_with_string_returns_string_as_is():
+    assert safe_serialize("hello") == "hello"
+    assert safe_serialize("12345") == "12345"
+
+
+def test_safe_serialize_with_int_returns_json_string():
+    assert safe_serialize(5) == "5"
+    assert safe_serialize(0) == "0"
+
+
+def test_safe_serialize_with_dict_returns_json_string():
+    data = {"key": "value", "num": 1}
+    assert safe_serialize(data) == json.dumps(data)
+
+
+def test_safe_serialize_with_bool_returns_json_string():
+    assert safe_serialize(True) == "true"
+    assert safe_serialize(False) == "false"
+
+
+def test_safe_serialize_with_custom_object_returns_str():
+    class CustomObject:
+        def __str__(self):
+            return "CustomObject"
+
+    obj = CustomObject()
+    assert safe_serialize(obj) == "CustomObject"
+
+
+def test_safe_serialize_with_none_returns_json_null():
+    assert safe_serialize(None) == "null"
+
+
+def test_safe_serialize_with_list_returns_json_string():
+    assert safe_serialize([1, 2, 3]) == "[1, 2, 3]"
+
+
+def test_safe_serialize_with_empty_string():
+    assert safe_serialize("") == ""