fix: raise a clear RuntimeError when inject runs without the DI interceptor

README.md

@@ -96,7 +96,7 @@ For an async server, pass `DIAioInterceptor(container)` to `grpc.aio.server(...)
 | `DIAioInterceptor(container)` | `grpc.aio.ServerInterceptor` for the async server. Same, with `close_async` |
 | `FromDI(dependency)` | Inert marker for `Annotated[T, FromDI(...)]` in servicer-method signatures; accepts a provider instance or a type |
 | `inject(method)` | Decorates a servicer method to resolve its `FromDI` parameters from the current RPC's child container; adapts to sync / async / async-generator methods |
-| `fetch_di_container()` | Returns the current RPC's child container (raises `LookupError` outside an RPC) |
+| `fetch_di_container()` | Returns the current RPC's child container (raises `RuntimeError` outside an RPC) |
 | `grpc_context_provider` | `ContextProvider` exposing `grpc.ServicerContext` at `Scope.REQUEST`; auto-registered by the interceptor |
 
 ## 📦 [PyPI](https://pypi.org/project/modern-di-grpc)

modern_di_grpc/main.py

@@ -23,9 +23,21 @@ def _build_child(container: Container, context: ServicerContext) -> Container:
     return child
 
 
+def _current_container() -> Container:
+    try:
+        return _request_container.get()
+    except LookupError:
+        msg = (
+            "No modern-di container found for this RPC. "
+            "Add DIInterceptor (sync server) or DIAioInterceptor (aio server) to the server's interceptors "
+            "so RPCs pass through it before using @inject or fetch_di_container."
+        )
+        raise RuntimeError(msg) from None
+
+
 def fetch_di_container() -> Container:
-    """Return the current RPC's child container. Raises ``LookupError`` outside an intercepted RPC."""
-    return _request_container.get()
+    """Return the current RPC's child container. Raises ``RuntimeError`` outside an intercepted RPC."""
+    return _current_container()
 
 
 def _ensure_context_provider(container: Container) -> None:
@@ -38,8 +50,7 @@ def _ensure_context_provider(container: Container) -> None:
 
 
 def _resolve(di_params: dict[str, integrations.Marker[typing.Any]]) -> dict[str, typing.Any]:
-    container = _request_container.get()
-    return integrations.resolve_markers(container, di_params)
+    return integrations.resolve_markers(_current_container(), di_params)
 
 
 def inject(func: typing.Callable[..., typing.Any]) -> typing.Callable[..., typing.Any]:

tests/test_inject.py

@@ -40,12 +40,26 @@ def test_fetch_di_container_returns_child() -> None:
 
 def test_fetch_di_container_raises_outside_rpc() -> None:
     def _call() -> None:
-        with pytest.raises(LookupError):
+        with pytest.raises(RuntimeError, match="DIInterceptor"):
             fetch_di_container()
 
     contextvars.copy_context().run(_call)  # guaranteed-unset ContextVar
 
 
+def test_inject_raises_without_interceptor() -> None:
+    @inject
+    def method(
+        _self: object, _request: str, _context: object, _app_res: typing.Annotated[AppResource, FromDI(AppResource)]
+    ) -> None:
+        pass  # pragma: no cover
+
+    def _call() -> None:
+        with pytest.raises(RuntimeError, match="DIInterceptor"):
+            method(object(), "req", object())
+
+    contextvars.copy_context().run(_call)  # guaranteed-unset ContextVar
+
+
 def test_inject_sync_resolves() -> None:
     @inject
     def method(

tests/test_sync.py

@@ -1,5 +1,6 @@
 import typing
 from collections.abc import Iterator
+from concurrent import futures
 
 import grpc
 import pytest
@@ -124,6 +125,20 @@ def test_unknown_method_returns_unimplemented(sync_channel: grpc.Channel) -> Non
     assert excinfo.value.code() == grpc.StatusCode.UNIMPLEMENTED  # ty: ignore[unresolved-attribute]
 
 
+def test_inject_without_interceptor_reports_missing_interceptor() -> None:
+    server = grpc.server(futures.ThreadPoolExecutor(max_workers=1))
+    greeter_pb2_grpc.add_GreeterServicer_to_server(Servicer(), server)
+    port = server.add_insecure_port("127.0.0.1:0")
+    server.start()
+    try:
+        with grpc.insecure_channel(f"127.0.0.1:{port}") as channel, pytest.raises(grpc.RpcError) as excinfo:
+            greeter_pb2_grpc.GreeterStub(channel).SayHello(HelloRequest(name="a"))
+    finally:
+        server.stop(0)
+    assert excinfo.value.code() == grpc.StatusCode.UNKNOWN  # ty: ignore[unresolved-attribute]
+    assert "DIInterceptor" in excinfo.value.details()  # ty: ignore[unresolved-attribute]
+
+
 async def test_app_finalizer_runs_on_root_close() -> None:
     app_teardowns.clear()
     container = Container(groups=[Dependencies])