TestClient basics in FastAPI - Mini Project: Build & Apply

TestClient basics

📖 Scenario: You are building a simple FastAPI app that returns a greeting message. You want to test this app using FastAPI's TestClient to make sure it works correctly.

🎯 Goal: Create a FastAPI app with one route, then write a test using TestClient to check the response status and content.

📋 What You'll Learn

Create a FastAPI app instance named app

Add a GET route /hello that returns JSON {"message": "Hello, FastAPI!"}

Create a TestClient instance using the app

Write a test function test_hello() that sends a GET request to /hello

Assert the response status code is 200

Assert the response JSON matches {"message": "Hello, FastAPI!"}

💡 Why This Matters

🌍 Real World

Testing web APIs is essential to ensure they work correctly before deployment. FastAPI's TestClient lets you simulate requests without running a server.

💼 Career

Backend developers often write automated tests for APIs to catch bugs early and maintain code quality. Knowing how to use TestClient is a key skill.

1

Create the FastAPI app with a greeting route

Create a FastAPI app instance called app. Add a GET route /hello that returns JSON {"message": "Hello, FastAPI!"}.

FastAPI

from fastapi import FastAPI

# Create FastAPI app instance

# Add GET route /hello

Need a hint?

Use FastAPI() to create the app. Use @app.get("/hello") decorator for the route. Return the dictionary with the message.

2

Import TestClient and create a client instance

Import TestClient from fastapi.testclient. Create a client instance using the app.

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get("/hello")
async def read_hello():
    return {"message": "Hello, FastAPI!"}

# Create TestClient instance

Need a hint?

Import TestClient and create client = TestClient(app).

3

Write a test function to request the /hello route

Write a test function called test_hello(). Inside it, use client.get("/hello") to send a GET request and store the response in response.

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get("/hello")
async def read_hello():
    return {"message": "Hello, FastAPI!"}

client = TestClient(app)

# Write test function test_hello

Need a hint?

Define def test_hello(): and call client.get("/hello") inside it.

4

Assert the response status and JSON content

Inside test_hello(), add assertions to check that response.status_code is 200 and response.json() equals {"message": "Hello, FastAPI!"}.

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get("/hello")
async def read_hello():
    return {"message": "Hello, FastAPI!"}

client = TestClient(app)

def test_hello():
    response = client.get("/hello")
    # Add assertions for status code and JSON content

Need a hint?

Use assert response.status_code == 200 and assert response.json() == {"message": "Hello, FastAPI!"}.