Testing Dependencies with Overrides¶

Overriding dependencies during testing¶

There are some scenarios where you might want to override a dependency during testing.

You don't want the original dependency to run (nor any of the sub-dependencies it might have).

Instead, you want to provide a different dependency that will be used only during tests (possibly only some specific tests), and will provide a value that can be used where the value of the original dependency was used.

Use cases: external service¶

An example could be that you have an external authentication provider that you need to call.

You send it a token and it returns an authenticated user.

This provider might be charging you per request, and calling it might take some extra time than if you had a fixed mock user for tests.

You probably want to test the external provider once, but not necessarily call it for every test that runs.

In this case, you can override the dependency that calls that provider, and use a custom dependency that returns a mock user, only for your tests.

Use theapp.dependency_overrides attribute¶

For these cases, yourFastAPI application has an attributeapp.dependency_overrides, it is a simpledict.

To override a dependency for testing, you put as a key the original dependency (a function), and as the value, your dependency override (another function).

And thenFastAPI will call that override instead of the original dependency.

fromtypingimportAnnotatedfromfastapiimportDepends,FastAPIfromfastapi.testclientimportTestClientapp=FastAPI()asyncdefcommon_parameters(q:str|None=None,skip:int=0,limit:int=100):return{"q":q,"skip":skip,"limit":limit}@app.get("/items/")asyncdefread_items(commons:Annotated[dict,Depends(common_parameters)]):return{"message":"Hello Items!","params":commons}@app.get("/users/")asyncdefread_users(commons:Annotated[dict,Depends(common_parameters)]):return{"message":"Hello Users!","params":commons}client=TestClient(app)asyncdefoverride_dependency(q:str|None=None):return{"q":q,"skip":5,"limit":10}app.dependency_overrides[common_parameters]=override_dependencydeftest_override_in_items():response=client.get("/items/")assertresponse.status_code==200assertresponse.json()=={"message":"Hello Items!","params":{"q":None,"skip":5,"limit":10},}deftest_override_in_items_with_q():response=client.get("/items/?q=foo")assertresponse.status_code==200assertresponse.json()=={"message":"Hello Items!","params":{"q":"foo","skip":5,"limit":10},}deftest_override_in_items_with_params():response=client.get("/items/?q=foo&skip=100&limit=200")assertresponse.status_code==200assertresponse.json()=={"message":"Hello Items!","params":{"q":"foo","skip":5,"limit":10},}

fromfastapiimportDepends,FastAPIfromfastapi.testclientimportTestClientapp=FastAPI()asyncdefcommon_parameters(q:str|None=None,skip:int=0,limit:int=100):return{"q":q,"skip":skip,"limit":limit}@app.get("/items/")asyncdefread_items(commons:dict=Depends(common_parameters)):return{"message":"Hello Items!","params":commons}@app.get("/users/")asyncdefread_users(commons:dict=Depends(common_parameters)):return{"message":"Hello Users!","params":commons}client=TestClient(app)asyncdefoverride_dependency(q:str|None=None):return{"q":q,"skip":5,"limit":10}app.dependency_overrides[common_parameters]=override_dependencydeftest_override_in_items():response=client.get("/items/")assertresponse.status_code==200assertresponse.json()=={"message":"Hello Items!","params":{"q":None,"skip":5,"limit":10},}deftest_override_in_items_with_q():response=client.get("/items/?q=foo")assertresponse.status_code==200assertresponse.json()=={"message":"Hello Items!","params":{"q":"foo","skip":5,"limit":10},}deftest_override_in_items_with_params():response=client.get("/items/?q=foo&skip=100&limit=200")assertresponse.status_code==200assertresponse.json()=={"message":"Hello Items!","params":{"q":"foo","skip":5,"limit":10},}

Then you can reset your overrides (remove them) by settingapp.dependency_overrides to be an emptydict:

app.dependency_overrides={}