Overriding dependencies in tests in FastAPI - Mini Project: Build & Apply

Overriding dependencies in tests

📖 Scenario: You are building a simple FastAPI app that depends on a function to get a message. For testing, you want to replace this function with a fake one to control the output.

🎯 Goal: Create a FastAPI app with a dependency function, then override this dependency in a test to return a fixed message.

📋 What You'll Learn

Create a dependency function called get_message that returns the string 'Hello from dependency!'

Create a FastAPI app instance called app

Create a GET endpoint at / that uses get_message as a dependency and returns a JSON with the message

Create a test client using TestClient from fastapi.testclient

Override the get_message dependency in the test to return 'Hello from override!'

Write a test function called test_read_main that uses the test client to call / and asserts the JSON response matches the overridden message

💡 Why This Matters

🌍 Real World

Overriding dependencies is useful when you want to test parts of your FastAPI app without relying on real external services or complex logic.

💼 Career

Many jobs require writing tests for APIs. Knowing how to override dependencies helps you write isolated, reliable tests for FastAPI applications.

1

Create the dependency function and FastAPI app

Create a function called get_message that returns the string 'Hello from dependency!'. Then create a FastAPI app instance called app.

FastAPI

# Your code here

Need a hint?

Use def get_message(): to define the function and app = FastAPI() to create the app.

2

Add a GET endpoint using the dependency

Add a GET endpoint at / to the app that uses get_message as a dependency with Depends. The endpoint should return a dictionary with the key message and the value from get_message.

FastAPI

from fastapi import FastAPI, Depends

# Your code here

Need a hint?

Use @app.get("/") to create the route and Depends(get_message) to inject the dependency.

3

Create a test client and override the dependency

Import TestClient from fastapi.testclient. Create a test client called client for the app. Then override the get_message dependency in app.dependency_overrides to a new function that returns 'Hello from override!'.

FastAPI

from fastapi.testclient import TestClient

# Your code here

Need a hint?

Create client = TestClient(app) and override with app.dependency_overrides[get_message] = override_get_message.

4

Write the test function to check the overridden dependency

Write a test function called test_read_main that uses the client to send a GET request to /. Assert that the response status code is 200 and the JSON response is {"message": "Hello from override!"}.

FastAPI

# Your code here

Need a hint?

Use client.get("/") and assert the status and JSON response.