Fix unstable operation IDs for multi-method routes

Make generated operation IDs deterministic when a route is registered with
multiple HTTP methods. This avoids depending on set iteration order and adds
a regression test to keep the behavior stable.

fastapi/utils.py

@@ -96,7 +96,7 @@ def generate_unique_id(route: "APIRoute") -> str:
     operation_id = f"{route.name}{route.path_format}"
     operation_id = re.sub(r"\W", "_", operation_id)
     assert route.methods
-    operation_id = f"{operation_id}_{list(route.methods)[0].lower()}"
+    operation_id = f"{operation_id}_{sorted(route.methods)[0].lower()}"
     return operation_id
 
 

tests/test_generate_unique_id_function.py

@@ -1,8 +1,10 @@
 import warnings
+from types import SimpleNamespace
 
 from fastapi import APIRouter, FastAPI
 from fastapi.routing import APIRoute
 from fastapi.testclient import TestClient
+from fastapi.utils import generate_unique_id
 from inline_snapshot import snapshot
 from pydantic import BaseModel
 
@@ -29,6 +31,31 @@ class Message(BaseModel):
     description: str
 
 
+class MethodsSet(set[str]):
+    def __init__(self, values: set[str], iteration_order: list[str]) -> None:
+        super().__init__(values)
+        self.iteration_order = iteration_order
+
+    def __iter__(self):
+        return iter(self.iteration_order)
+
+
+def test_generate_unique_id_with_multiple_methods_is_stable():
+    route_a = SimpleNamespace(
+        name="handler",
+        path_format="/items",
+        methods=MethodsSet({"POST", "DELETE"}, ["POST", "DELETE"]),
+    )
+    route_b = SimpleNamespace(
+        name="handler",
+        path_format="/items",
+        methods=MethodsSet({"POST", "DELETE"}, ["DELETE", "POST"]),
+    )
+
+    assert generate_unique_id(route_a) == "handler_items_delete"
+    assert generate_unique_id(route_a) == generate_unique_id(route_b)
+
+
 def test_top_level_generate_unique_id():
     app = FastAPI(generate_unique_id_function=custom_generate_unique_id)
     router = APIRouter()