Handle Fn::Transforms inside Mappings

src/cfnlint/context/context.py

@@ -384,11 +384,15 @@ class _MappingSecondaryKey:
         init=False, default_factory=dict
     )
     instance: InitVar[Any]
+    is_transform: bool = field(init=False, default=False)
 
     def __post_init__(self, instance) -> None:
         if not isinstance(instance, dict):
             raise ValueError("Secondary keys must be a object")
         for k, v in instance.items():
+            if k == "Fn::Transform":
+                self.is_transform = True
+                continue
             if isinstance(v, (str, list, int, float)):
                 self.keys[k] = v
             else:
@@ -408,12 +412,16 @@ class Map:
 
     keys: dict[str, _MappingSecondaryKey] = field(init=False, default_factory=dict)
     resource: InitVar[Any]
+    is_transform: bool = field(init=False, default=False)
 
     def __post_init__(self, mapping) -> None:
         if not isinstance(mapping, dict):
             raise ValueError("Mapping must be a object")
         for k, v in mapping.items():
-            self.keys[k] = _MappingSecondaryKey(v)
+            if k == "Fn::Transform":
+                self.is_transform = True
+            else:
+                self.keys[k] = _MappingSecondaryKey(v)
 
     def find_in_map(self, top_key: str, secondary_key: str) -> Iterator[Any]:
         if top_key not in self.keys:

src/cfnlint/jsonschema/_resolvers_cfn.py

@@ -85,6 +85,8 @@ def find_in_map(validator: Validator, instance: Any) -> ResolutionResult:
 
         top_key = map.keys.get(instance[1])
         if top_key is None:
+            if map.is_transform:
+                return
             if not default_value_found:
                 yield None, validator, ValidationError(
                     (
@@ -98,6 +100,8 @@ def find_in_map(validator: Validator, instance: Any) -> ResolutionResult:
 
         value = top_key.keys.get(instance[2])
         if value is None:
+            if top_key.is_transform:
+                return
             if not default_value_found:
                 yield value, validator, ValidationError(
                     (

test/unit/module/context/test_mappings.py

@@ -24,3 +24,13 @@ def test_mapping_value(name, get_key_1, get_key_2, expected):
             list(mapping.find_in_map(get_key_1, get_key_2))
     else:
         assert list(mapping.find_in_map(get_key_1, get_key_2)) == expected
+
+
+def test_transforms():
+    mapping = Map({"A": {"Fn::Transform": "C"}})
+
+    assert mapping.keys.get("A").is_transform is True
+
+    mapping = Map({"Fn::Transform": {"B": "C"}})
+
+    assert mapping.is_transform is True

test/unit/module/jsonschema/test_resolvers_cfn.py

@@ -240,7 +240,10 @@ def test_invalid_functions(name, instance, response):
                     None,
                     deque([]),
                     ValidationError(
-                        ("'bar' is not one of ['foo']"),
+                        (
+                            "'bar' is not one of ['foo', "
+                            "'transformFirstKey', 'transformSecondKey']"
+                        ),
                         path=deque(["Fn::FindInMap", 0]),
                     ),
                 )
@@ -304,6 +307,16 @@ def test_invalid_functions(name, instance, response):
             {"Fn::FindInMap": ["foo", "first", "third", {"DefaultValue": "default"}]},
             [("default", deque([4, "DefaultValue"]), None)],
         ),
+        (
+            "Valid FindInMap with a transform on first key",
+            {"Fn::FindInMap": ["transformFirstKey", "first", "third"]},
+            [],
+        ),
+        (
+            "Valid FindInMap with a transform on second key",
+            {"Fn::FindInMap": ["transformSecondKey", "first", "third"]},
+            [],
+        ),
         (
             "Valid Sub with a resolvable values",
             {"Fn::Sub": ["${a}-${b}", {"a": "foo", "b": "bar"}]},
@@ -319,6 +332,8 @@ def test_invalid_functions(name, instance, response):
 def test_valid_functions(name, instance, response):
     context = Context()
     context.mappings["foo"] = Map({"first": {"second": "bar"}})
+    context.mappings["transformFirstKey"] = Map({"Fn::Transform": {"second": "bar"}})
+    context.mappings["transformSecondKey"] = Map({"first": {"Fn::Transform": "bar"}})
 
     _resolve(name, instance, response, context=context)