Fix RouterPathParamsModifiedError when renaming endpoints with {api_version} prefix

When api_version_location="path", the api_version param appears in route
paths but not in dependant.path_params, causing a false positive when
validating path param consistency on endpoint renames.

Thread api_version_parameter_name through generate_versioned_routers and
_EndpointTransformer down to _apply_endpoint_had_instruction, which now
discards the version param directly instead of inferring it via regex.

Fixes #357

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: zmievsa

cadwyn/applications.py

@@ -215,9 +215,11 @@ def __init__(
         if api_version_location == "custom_header":
             self._api_version_manager = HeaderVersionManager(api_version_parameter_name=api_version_parameter_name)
             self._api_version_fastapi_depends_class = fastapi.Header
+            self._api_version_path_param_name: Union[str, None] = None
         elif api_version_location == "path":
             self._api_version_manager = URLVersionManager(possible_version_values=self.versions._version_values_set)
             self._api_version_fastapi_depends_class = fastapi.Path
+            self._api_version_path_param_name = api_version_parameter_name
         else:
             assert_never(api_version_location)
         # TODO: Add a test validating the error message when there are no versions
@@ -265,6 +267,7 @@ def _cadwyn_initialize(self) -> None:
                 self._latest_version_router,
                 webhooks=self.webhooks,
                 versions=self.versions,
+                api_version_parameter_name=self._api_version_path_param_name,
             )
         except RecursionError as e:
             raise ImportError(

cadwyn/route_generation.py

@@ -76,10 +76,11 @@ def generate_versioned_routers(
     versions: VersionBundle,
     *,
     webhooks: Union[_WR, None] = None,
+    api_version_parameter_name: Union[str, None] = None,
 ) -> GeneratedRouters[_R, _WR]:
     if webhooks is None:
         webhooks = cast("_WR", APIRouter())
-    return _EndpointTransformer(router, versions, webhooks).transform()
+    return _EndpointTransformer(router, versions, webhooks, api_version_parameter_name).transform()
 
 
 class VersionedAPIRouter(fastapi.routing.APIRouter):
@@ -128,11 +129,18 @@ def copy_route(route: _RouteT) -> _RouteT:
 
 
 class _EndpointTransformer(Generic[_R, _WR]):
-    def __init__(self, parent_router: _R, versions: VersionBundle, webhooks: _WR) -> None:
+    def __init__(
+        self,
+        parent_router: _R,
+        versions: VersionBundle,
+        webhooks: _WR,
+        api_version_parameter_name: Union[str, None] = None,
+    ) -> None:
         super().__init__()
         self.parent_router = parent_router
         self.versions = versions
         self.parent_webhooks_router = webhooks
+        self.api_version_parameter_name = api_version_parameter_name
         self.schema_generators = generate_versioned_models(versions)
 
         self.routes_that_never_existed = [
@@ -425,7 +433,12 @@ def _apply_endpoint_changes_to_router(  # noqa: C901
                 elif isinstance(instruction, EndpointHadInstruction):
                     for original_route in original_routes:
                         methods_to_which_we_applied_changes |= original_route.methods
-                        _apply_endpoint_had_instruction(version_change.__name__, instruction, original_route)
+                        _apply_endpoint_had_instruction(
+                            version_change.__name__,
+                            instruction,
+                            original_route,
+                            self.api_version_parameter_name,
+                        )
                     err = (
                         'Endpoint "{endpoint_methods} {endpoint_path}" you tried to change in'
                         ' "{version_change_name}" doesn\'t exist'
@@ -484,6 +497,7 @@ def _apply_endpoint_had_instruction(
     version_change_name: str,
     instruction: EndpointHadInstruction,
     original_route: APIRoute,
+    api_version_parameter_name: Union[str, None] = None,
 ):
     for attr_name in instruction.attributes.__dataclass_fields__:
         attr = getattr(instruction.attributes, attr_name)
@@ -499,6 +513,8 @@ def _apply_endpoint_had_instruction(
             if attr_name == "path":
                 original_path_params = {p.alias for p in original_route.dependant.path_params}
                 new_path_params = set(re.findall("{(.*?)}", attr))
+                if api_version_parameter_name is not None:
+                    new_path_params.discard(api_version_parameter_name)
                 if new_path_params != original_path_params:
                     raise RouterPathParamsModifiedError(
                         f'When altering the path of "{list(original_route.methods)} {original_route.path}" '

tests/test_router_generation.py

@@ -253,6 +253,40 @@ def test__endpoint_had_another_path_variable(
         )
 
 
+def test__endpoint_had__path_rename_with_api_version_in_prefix__should_not_raise():
+    app_router = VersionedAPIRouter(prefix="/{api_version}")
+    sub_router = VersionedAPIRouter()
+
+    @sub_router.get("")
+    async def list_items():
+        return 83
+
+    app_router.include_router(sub_router, prefix="/new-name")
+
+    class RenameEndpoint(VersionChange):
+        description = "Rename /old-name to /new-name"
+        instructions_to_migrate_to_previous_version = (
+            endpoint("/{api_version}/new-name", ["GET"]).had(path="/{api_version}/old-name"),
+        )
+
+    versions = VersionBundle(
+        Version("v2", RenameEndpoint),
+        Version("v1"),
+    )
+
+    cadwyn_app = Cadwyn(
+        versions=versions,
+        api_version_location="path",
+        api_version_parameter_name="api_version",
+        api_version_format="string",
+    )
+    cadwyn_app.generate_and_include_versioned_routers(app_router)
+
+    client_v2 = TestClient(cadwyn_app)
+    assert client_v2.get("/v2/new-name").json() == 83
+    assert client_v2.get("/v1/old-name").json() == 83
+
+
 def test__endpoint_had__another_path_with_the_other_migration_at_the_same_time__should_require_old_name(
     create_versioned_app: CreateVersionedApp,
 ):