Overriding dependencies in tests in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when overriding a dependency in FastAPI test client?

Given this FastAPI app with a dependency that returns a user ID, and a test that overrides this dependency, what will the test client receive as the response?

FastAPI

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient

app = FastAPI()

def get_user_id():
    return 42

@app.get("/user")
async def read_user(user_id: int = Depends(get_user_id)):
    return {"user_id": user_id}

client = TestClient(app)

def override_get_user_id():
    return 99

app.dependency_overrides[get_user_id] = override_get_user_id

response = client.get("/user")
print(response.json())

A {"user_id": 42}

B {"user_id": 99}

C {"user_id": null}

D Raises a RuntimeError

💡 Hint

Think about what the override function returns and how FastAPI uses dependency overrides in tests.

The test overrides the get_user_id dependency with override_get_user_id which returns 99. So the endpoint returns {"user_id": 99}.

📝 Syntax

intermediate

Which option correctly overrides a FastAPI dependency in a test?

Choose the correct way to override the get_db dependency in a FastAPI test.

FastAPI

def get_db():
    return "real_db"

app = FastAPI()

@app.get("/items")
async def read_items(db=Depends(get_db)):
    return {"db": db}

client = TestClient(app)

A app.dependency_overrides[get_db] = lambda: "test_db"

B app.override_dependency(get_db, lambda: "test_db")

C app.dependency_overrides[get_db] = "test_db"

D app.dependency_overrides[get_db()] = lambda: "test_db"

💡 Hint

Remember the syntax for dependency_overrides dictionary keys and values.

Option A correctly assigns a lambda function to override the get_db dependency. Option A uses a non-existent method. Option A uses a call instead of the function as key. Option A assigns a string instead of a callable.

🔧 Debug

advanced

Why does this FastAPI test fail with a 422 error despite overriding the dependency?

Consider this FastAPI endpoint and test. The test overrides the dependency but the response status code is 422. Why?

FastAPI

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient

app = FastAPI()

def get_token():
    return "real_token"

@app.get("/secure")
async def secure_route(token: str = Depends(get_token)):
    return {"token": token}

client = TestClient(app)

def override_token():
    return None

app.dependency_overrides[get_token] = override_token

response = client.get("/secure")
print(response.status_code)

A Because the endpoint requires authentication headers missing in the test

B Because the dependency override was not applied correctly

C Because the overridden dependency returns None, which FastAPI rejects for a required str parameter

D Because the test client is not instantiated properly

💡 Hint

Think about how FastAPI validates dependency return values for required parameters.

The overridden dependency returns None, but the endpoint expects a non-optional string. FastAPI raises a 422 Unprocessable Entity error because None is invalid for a required str parameter.

❓ state_output

advanced

What is the value of the global variable after running this FastAPI test with dependency override?

Given this code, what is the value of state['count'] after the test client calls the endpoint?

FastAPI

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient

app = FastAPI()
state = {"count": 0}

def increment():
    state["count"] += 1
    return state["count"]

@app.get("/count")
async def get_count(count=Depends(increment)):
    return {"count": count}

client = TestClient(app)

def override_increment():
    state["count"] += 10
    return state["count"]

app.dependency_overrides[increment] = override_increment

client.get("/count")

A 10

B 0

C 1

D 11

💡 Hint

Consider how the override changes the state and what the original function did before.

The original increment adds 1 to count. The override adds 10 instead. Since state['count'] starts at 0, after the override runs once, it becomes 10. The original increment was never called, so count is 10 after the override. The endpoint returns 10, so state['count'] is 10 after the call.

🧠 Conceptual

expert

Which statement about FastAPI dependency overrides in tests is TRUE?

Choose the correct statement about how FastAPI handles dependency overrides during testing.

A Dependency overrides can only override async dependencies, not sync ones

B Dependency overrides only apply to the first request made by the test client

C Dependency overrides automatically reset after each test function when using TestClient

D Dependency overrides affect all requests globally until manually cleared or the app is restarted

💡 Hint

Think about the scope and lifetime of app.dependency_overrides.

Dependency overrides are stored in a dictionary on the app instance and affect all requests globally until cleared. They do not reset automatically after each test or only apply once. Both sync and async dependencies can be overridden.