📝 Update testing events documentation (#13259)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Motov Yurii <109919500+YuriiMotov@users.noreply.github.com>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs_src/app_testing/tutorial004.py

@@ -0,0 +1,43 @@
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+items = {}
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    items["foo"] = {"name": "Fighters"}
+    items["bar"] = {"name": "Tenders"}
+    yield
+    # clean up items
+    items.clear()
+
+
+app = FastAPI(lifespan=lifespan)
+
+
+@app.get("/items/{item_id}")
+async def read_items(item_id: str):
+    return items[item_id]
+
+
+def test_read_items():
+    # Before the lifespan starts, "items" is still empty
+    assert items == {}
+
+    with TestClient(app) as client:
+        # Inside the "with TestClient" block, the lifespan starts and items added
+        assert items == {"foo": {"name": "Fighters"}, "bar": {"name": "Tenders"}}
+
+        response = client.get("/items/foo")
+        assert response.status_code == 200
+        assert response.json() == {"name": "Fighters"}
+
+        # After the requests is done, the items are still there
+        assert items == {"foo": {"name": "Fighters"}, "bar": {"name": "Tenders"}}
+
+    # The end of the "with TestClient" block simulates terminating the app, so
+    # the lifespan ends and items are cleaned up
+    assert items == {}