localstack · MEPalma · Jun 2, 2025 · May 19, 2025 · May 20, 2025 · May 21, 2025
@@ -386,6 +386,7 @@ def __init__(self, scope: Scope, value: Any):
 FnEqualsKey: Final[str] = "Fn::Equals"
 FnFindInMapKey: Final[str] = "Fn::FindInMap"
 FnSubKey: Final[str] = "Fn::Sub"
+FnTransform: Final[str] = "Fn::Transform"
 INTRINSIC_FUNCTIONS: Final[set[str]] = {
     RefKey,
     FnIfKey,
@@ -395,6 +396,7 @@ def __init__(self, scope: Scope, value: Any):
     FnGetAttKey,
     FnFindInMapKey,
     FnSubKey,
+    FnTransform,
 }
 
 

@@ -3,6 +3,11 @@
 import re
 from typing import Any, Final, Generic, Optional, TypeVar
 
+from localstack.services.cloudformation.engine.transformers import (
+    Transformer,
+    execute_macro,
+    transformers,
+)
 from localstack.services.cloudformation.engine.v2.change_set_model import (
     ChangeSetEntity,
     ChangeType,
@@ -30,6 +35,7 @@
 from localstack.services.cloudformation.engine.v2.change_set_model_visitor import (
     ChangeSetModelVisitor,
 )
+from localstack.services.cloudformation.stores import get_cloudformation_store
 from localstack.services.cloudformation.v2.entities import ChangeSet
 from localstack.utils.aws.arns import get_partition
 from localstack.utils.urls import localstack_host
@@ -490,6 +496,78 @@ def visit_node_intrinsic_function_fn_not(
         # Implicit change type computation.
         return PreprocEntityDelta(before=before, after=after)
 
+    def _compute_fn_transform(self, args: dict[str, Any]) -> Any:
+        # TODO: add typing to arguments before this level.
+        # TODO: add schema validation
+        # TODO: add support for other transform types
+
+        account_id = self._change_set.account_id
+        region_name = self._change_set.region_name
+        transform_name: str = args.get("Name")
+        if not isinstance(transform_name, str):
+            raise RuntimeError("Invalid or missing Fn::Transform 'Name' argument")
+        transform_parameters: dict = args.get("Parameters")
+        if not isinstance(transform_parameters, dict):
+            raise RuntimeError("Invalid or missing Fn::Transform 'Parameters' argument")
+
+        if transform_name in transformers:
+            # TODO: port and refactor this 'transformers' logic to this package.
+            builtin_transformer_class = transformers[transform_name]
+            builtin_transformer: Transformer = builtin_transformer_class()
+            transform_output: Any = builtin_transformer.transform(
+                account_id=account_id, region_name=region_name, parameters=transform_parameters
+            )
+            return transform_output
+
+        macros_store = get_cloudformation_store(
+            account_id=account_id, region_name=region_name
+        ).macros
+        if transform_name in macros_store:
+            # TODO: this formatting of stack parameters is odd but required to integrate with v1 execute_macro util.
+            #  consider porting this utils and passing the plain list of parameters instead.
+            stack_parameters = {
+                parameter["ParameterKey"]: parameter
+                for parameter in self._change_set.stack.parameters
+            }
+            transform_output: Any = execute_macro(
+                account_id=account_id,
+                region_name=region_name,
+                parsed_template=dict(),  # TODO: review the requirements for this argument.
+                macro=args,  # TODO: review support for non dict bindings (v1).
+                stack_parameters=stack_parameters,
+                transformation_parameters=transform_parameters,
+                is_intrinsic=True,
+            )
+            return transform_output
+
+        raise RuntimeError(
+            f"Unsupported transform function '{transform_name}' in '{self._change_set.stack.stack_name}'"
+        )
+
+    def visit_node_intrinsic_function_fn_transform(
+        self, node_intrinsic_function: NodeIntrinsicFunction
+    ) -> PreprocEntityDelta:
+        arguments_delta = self.visit(node_intrinsic_function.arguments)
+        arguments_before = arguments_delta.before
+        arguments_after = arguments_delta.after
+
+        # TODO: review the use of cache in self.precessed from the 'before' run to
+        #  ensure changes to the lambda (such as after UpdateFunctionCode) do not
+        #  generalise tot he before value at this depth (thus making it seems as
+        #  though for this transformation before==after). Another options may be to
+        #  have specialised caching for transformations.
+
+        # TODO: add tests to review the behaviour of CFN with changes to transformation
+        #  function code and no changes to the template.
+
+        before = None
+        if arguments_before:
+            before = self._compute_fn_transform(args=arguments_before)
+        after = None
+        if arguments_after:
+            after = self._compute_fn_transform(args=arguments_after)
+        return PreprocEntityDelta(before=before, after=after)
+
     def visit_node_intrinsic_function_fn_sub(
         self, node_intrinsic_function: NodeIntrinsicFunction
     ) -> PreprocEntityDelta:
@@ -532,6 +610,8 @@ def _compute_sub(args: str | list[Any], select_before: bool = False) -> str:
                         template_variable_value = (
                             reference_delta.before if select_before else reference_delta.after
                         )
+                        if isinstance(template_variable_value, PreprocResource):
+                            template_variable_value = template_variable_value.logical_id
                     except RuntimeError:
                         raise RuntimeError(
                             f"Undefined variable name in Fn::Sub string template '{template_variable_name}'"

@@ -113,6 +113,11 @@ def visit_node_intrinsic_function_fn_equals(
     ):
         self.visit_children(node_intrinsic_function)
 
+    def visit_node_intrinsic_function_fn_transform(
+        self, node_intrinsic_function: NodeIntrinsicFunction
+    ):
+        self.visit_children(node_intrinsic_function)
+
     def visit_node_intrinsic_function_fn_sub(self, node_intrinsic_function: NodeIntrinsicFunction):
         self.visit_children(node_intrinsic_function)
 

@@ -283,7 +283,7 @@ def test_update_stack_with_same_template_withoutchange(
 
         snapshot.match("no_change_exception", ctx.value.response)
 
-    @pytest.mark.skip(reason="CFNV2:Transform")
+    @pytest.mark.skip(reason="CFNV2:Other")
     @markers.aws.validated
     def test_update_stack_with_same_template_withoutchange_transformation(
         self, deploy_cfn_template, aws_client

@@ -12,8 +12,6 @@
     reason="Only targeting the new engine",
 )
 
-pytestmark = pytest.mark.skip(reason="CFNV2:Transform")
-
 
 @markers.aws.validated
 @markers.snapshot.skip_snapshot_verify(paths=["$..tags"])
@@ -73,6 +71,12 @@ def test_duplicate_resources(deploy_cfn_template, s3_bucket, snapshot, aws_clien
     snapshot.match("api-resources", resources)
 
 
+@pytest.mark.skip(
+    reason=(
+        "CFNV2:AWS::Include the transformation is run however the "
+        "physical resource id for the resource is not available"
+    )
+)
 @markers.aws.validated
 def test_transformer_property_level(deploy_cfn_template, s3_bucket, aws_client, snapshot):
     api_spec = textwrap.dedent("""
@@ -125,6 +129,12 @@ def test_transformer_property_level(deploy_cfn_template, s3_bucket, aws_client,
     snapshot.match("processed_template", processed_template)
 
 
+@pytest.mark.skip(
+    reason=(
+        "CFNV2:AWS::Include the transformation is run however the "
+        "physical resource id for the resource is not available"
+    )
+)
 @markers.aws.validated
 def test_transformer_individual_resource_level(deploy_cfn_template, s3_bucket, aws_client):
     api_spec = textwrap.dedent("""