perf: pass template dict directly to Template, skip yaml round-trip

Template.__init__ now accepts an optional template_dict parameter.
When provided, it skips both file reading and yaml_parse, avoiding
the deepcopy → yaml_dump → yaml_parse round-trip that package_context
and artifact_exporter were doing.

For large templates with many Fn::ForEach expansions, this eliminates
two O(n) serialization passes (yaml_dump + yaml_parse) per export.

Author: bnusunny

samcli/commands/package/package_context.py

@@ -193,17 +193,17 @@ def _export(self, template_path, use_json):
 
         uses_language_extensions = result.had_language_extensions
         dynamic_properties = result.dynamic_artifact_properties
-        template_dict_for_export = copy.deepcopy(result.expanded_template)
 
-        # Create Template with the (possibly expanded) template
+        # Create Template with the (possibly expanded) template dict directly,
+        # avoiding a yaml_dump → yaml_parse round-trip.
         template = Template(
             template_path,
             os.getcwd(),
             self.uploaders,
             self.code_signer,
             normalize_template=True,
             normalize_parameters=True,
-            template_str=yaml_dump(template_dict_for_export),
+            template_dict=copy.deepcopy(result.expanded_template),
             parameter_values=parameter_values,
         )
         # Set template_dir since we're using template_str

samcli/lib/package/artifact_exporter.py

@@ -223,7 +223,7 @@ def do_export(self, resource_id, resource_dict, parent_dir):
                 normalize_template=True,
                 normalize_parameters=True,
                 parent_stack_id=resource_id,
-                template_str=yaml_dump(copy.deepcopy(result.expanded_template)),
+                template_dict=copy.deepcopy(result.expanded_template),
                 parameter_values=parameter_values,
             )
             template.template_dir = child_template_dir
@@ -351,11 +351,17 @@ def __init__(
         normalize_parameters: bool = False,
         parent_stack_id: str = "",
         parameter_values: Optional[Dict] = None,
+        template_dict: Optional[Dict] = None,
     ):
         """
         Reads the template and makes it ready for export
         """
-        if not template_str:
+        if template_dict is not None:
+            # Pre-parsed dict provided — skip file reading and YAML parsing
+            self.template_dict = template_dict
+        elif template_str:
+            self.template_dict = yaml_parse(template_str)
+        else:
             if not (is_local_folder(parent_dir) and os.path.isabs(parent_dir)):
                 raise ValueError("parent_dir parameter must be an absolute path to a folder {0}".format(parent_dir))
 
@@ -367,7 +373,7 @@ def __init__(
 
             self.template_dir = template_dir
             self.code_signer = code_signer
-        self.template_dict = yaml_parse(template_str)
+            self.template_dict = yaml_parse(template_str)
         if normalize_template:
             ResourceMetadataNormalizer.normalize(self.template_dict, normalize_parameters)
         self.resources_to_export = resources_to_export

tests/unit/lib/package/test_artifact_exporter.py

@@ -2424,7 +2424,7 @@ def test_export_cloudformation_stack_with_language_extensions(self, TemplateMock
             # Template should have been called with the expanded template_str
             TemplateMock.assert_called_once()
             call_kwargs = TemplateMock.call_args
-            self.assertIn("template_str", call_kwargs.kwargs)
+            self.assertIn("template_dict", call_kwargs.kwargs)
 
             template_instance_mock.export.assert_called_once_with()
             self.s3_uploader_mock.upload.assert_called_once_with(mock.ANY, mock.ANY)