Sync fastapi docs from b5ca1324 on 2025-12-07

tests/test_dependency_contextvars.py

@@ -0,0 +1,51 @@
+from contextvars import ContextVar
+from typing import Any, Awaitable, Callable, Dict, Optional
+
+from fastapi import Depends, FastAPI, Request, Response
+from fastapi.testclient import TestClient
+
+legacy_request_state_context_var: ContextVar[Optional[Dict[str, Any]]] = ContextVar(
+    "legacy_request_state_context_var", default=None
+)
+
+app = FastAPI()
+
+
+async def set_up_request_state_dependency():
+    request_state = {"user": "deadpond"}
+    contextvar_token = legacy_request_state_context_var.set(request_state)
+    yield request_state
+    legacy_request_state_context_var.reset(contextvar_token)
+
+
+@app.middleware("http")
+async def custom_middleware(
+    request: Request, call_next: Callable[[Request], Awaitable[Response]]
+):
+    response = await call_next(request)
+    response.headers["custom"] = "foo"
+    return response
+
+
+@app.get("/user", dependencies=[Depends(set_up_request_state_dependency)])
+def get_user():
+    request_state = legacy_request_state_context_var.get()
+    assert request_state
+    return request_state["user"]
+
+
+client = TestClient(app)
+
+
+def test_dependency_contextvars():
+    """
+    Check that custom middlewares don't affect the contextvar context for dependencies.
+
+    The code before yield and the code after yield should be run in the same contextvar
+    context, so that request_state_context_var.reset(contextvar_token).
+
+    If they are run in a different context, that raises an error.
+    """
+    response = client.get("/user")
+    assert response.json() == "deadpond"
+    assert response.headers["custom"] == "foo"