✨ Implement dependency value cache per request (#292)

* ✨ Add dependency cache, with support for disabling it

* ✅ Add tests for dependency cache

* 📝 Add docs about dependency value caching

tests/test_dependency_cache.py

@@ -0,0 +1,68 @@
+from fastapi import Depends, FastAPI
+from starlette.testclient import TestClient
+
+app = FastAPI()
+
+counter_holder = {"counter": 0}
+
+
+async def dep_counter():
+    counter_holder["counter"] += 1
+    return counter_holder["counter"]
+
+
+async def super_dep(count: int = Depends(dep_counter)):
+    return count
+
+
+@app.get("/counter/")
+async def get_counter(count: int = Depends(dep_counter)):
+    return {"counter": count}
+
+
+@app.get("/sub-counter/")
+async def get_sub_counter(
+    subcount: int = Depends(super_dep), count: int = Depends(dep_counter)
+):
+    return {"counter": count, "subcounter": subcount}
+
+
+@app.get("/sub-counter-no-cache/")
+async def get_sub_counter_no_cache(
+    subcount: int = Depends(super_dep),
+    count: int = Depends(dep_counter, use_cache=False),
+):
+    return {"counter": count, "subcounter": subcount}
+
+
+client = TestClient(app)
+
+
+def test_normal_counter():
+    counter_holder["counter"] = 0
+    response = client.get("/counter/")
+    assert response.status_code == 200
+    assert response.json() == {"counter": 1}
+    response = client.get("/counter/")
+    assert response.status_code == 200
+    assert response.json() == {"counter": 2}
+
+
+def test_sub_counter():
+    counter_holder["counter"] = 0
+    response = client.get("/sub-counter/")
+    assert response.status_code == 200
+    assert response.json() == {"counter": 1, "subcounter": 1}
+    response = client.get("/sub-counter/")
+    assert response.status_code == 200
+    assert response.json() == {"counter": 2, "subcounter": 2}
+
+
+def test_sub_counter_no_cache():
+    counter_holder["counter"] = 0
+    response = client.get("/sub-counter-no-cache/")
+    assert response.status_code == 200
+    assert response.json() == {"counter": 2, "subcounter": 1}
+    response = client.get("/sub-counter-no-cache/")
+    assert response.status_code == 200
+    assert response.json() == {"counter": 4, "subcounter": 3}