fix: raise a clear RuntimeError when FromDI resolves without setup_di (#51)

* fix: raise a clear RuntimeError when FromDI resolves without setup_di

* docs: note the RuntimeError FromDI raises without setup_di

Author: lesnik512

README.md

@@ -74,8 +74,8 @@ The framework `Request` / `WebSocket` are resolvable within DI via the pre-built
 | Symbol | Description |
 |---|---|
 | `setup_di(app, container)` | Stores the container on `app.state` and composes the container's open/close into the app lifespan (nesting inside any existing `lifespan=`) |
-| `FromDI(provider, *, use_cache=True)` | FastAPI `Depends` that resolves a provider (or type) from the per-connection child container |
-| `fetch_di_container(app)` | Returns the app-scoped container from `app.state` |
+| `FromDI(provider, *, use_cache=True)` | FastAPI `Depends` that resolves a provider (or type) from the per-connection child container. Raises `RuntimeError` naming `setup_di` when a request reaches it without `setup_di` called |
+| `fetch_di_container(app)` | Returns the app-scoped container from `app.state`. Raises `RuntimeError` naming `setup_di` when called without `setup_di` called |
 | `build_di_container(connection)` | FastAPI `Depends` callable that yields the per-connection child container — `REQUEST` scope for an HTTP request, `SESSION` scope for a WebSocket |
 | `fastapi_request_provider` | `ContextProvider` for the current `fastapi.Request` |
 | `fastapi_websocket_provider` | `ContextProvider` for the current `fastapi.WebSocket` |

modern_di_fastapi/main.py

@@ -22,7 +22,14 @@
 
 
 def fetch_di_container(app_: fastapi.FastAPI) -> Container:
-    return typing.cast(Container, app_.state.di_container)
+    try:
+        return typing.cast(Container, app_.state.di_container)
+    except AttributeError:
+        msg = (
+            "No modern-di container found on the app. "
+            "Call setup_di(app, container) before using FromDI or fetch_di_container."
+        )
+        raise RuntimeError(msg) from None
 
 
 def _compose_lifespan(original: Lifespan[fastapi.FastAPI]) -> Lifespan[fastapi.FastAPI]:

tests/test_routes.py

@@ -1,6 +1,7 @@
 import typing
 
 import fastapi
+import pytest
 from modern_di import Container
 from starlette import status
 from starlette.testclient import TestClient
@@ -48,3 +49,13 @@ async def read_root(
     response = client.get("/")
     assert response.status_code == status.HTTP_200_OK
     assert response.json() is None
+
+
+def test_from_di_without_setup_di_raises_clear_error() -> None:
+    app = fastapi.FastAPI()
+
+    @app.get("/")
+    async def read_root(instance: typing.Annotated[SimpleCreator, FromDI(SimpleCreator)]) -> None: ...
+
+    with TestClient(app=app) as client, pytest.raises(RuntimeError, match=r"setup_di\(app, container\)"):
+        client.get("/")