Testing authentication in FastAPI - Mini Project: Build & Apply

Testing authentication

📖 Scenario: You are building a simple FastAPI app that requires users to log in. You want to write tests to check if the authentication works correctly.

🎯 Goal: Create a test that checks if the login endpoint returns a success status when given correct credentials.

📋 What You'll Learn

Create a FastAPI app with a login endpoint

Add a test client to send requests to the app

Write a test function to check successful login

Use assertions to verify the response status code

💡 Why This Matters

🌍 Real World

Testing authentication endpoints is essential for any web app that requires user login to ensure security and correct behavior.

💼 Career

Backend developers and QA engineers often write tests for authentication to prevent security issues and bugs in production.

1

Create the FastAPI app with a login endpoint

Create a FastAPI app called app and add a POST endpoint /login that accepts JSON with username and password. Return {"message": "Login successful"} if username is "user1" and password is "pass123", else return {"message": "Invalid credentials"}.

FastAPI

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI()

# Your code here

Need a hint?

Use FastAPI() to create the app. Define a Pydantic model for the login data. Use @app.post("/login") to create the endpoint.

2

Set up the test client

Import TestClient from fastapi.testclient and create a test client called client for the app.

FastAPI

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
from fastapi.testclient import TestClient

app = FastAPI()

class LoginData(BaseModel):
    username: str
    password: str

@app.post("/login")
async def login(data: LoginData):
    if data.username == "user1" and data.password == "pass123":
        return {"message": "Login successful"}
    else:
        return {"message": "Invalid credentials"}

# Your code here

Need a hint?

Import TestClient and create it by passing app.

3

Write a test function for successful login

Write a test function called test_login_success that uses client.post to send JSON {"username": "user1", "password": "pass123"} to /login. Store the response in a variable called response.

FastAPI

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
from fastapi.testclient import TestClient

app = FastAPI()

class LoginData(BaseModel):
    username: str
    password: str

@app.post("/login")
async def login(data: LoginData):
    if data.username == "user1" and data.password == "pass123":
        return {"message": "Login successful"}
    else:
        return {"message": "Invalid credentials"}

client = TestClient(app)

def test_login_success():
    # Your code here

Need a hint?

Define a function named test_login_success. Use client.post with json= to send the login data.

4

Add assertion to check response status code

Inside the test_login_success function, add an assertion that checks if response.status_code is 200.

FastAPI

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
from fastapi.testclient import TestClient

app = FastAPI()

class LoginData(BaseModel):
    username: str
    password: str

@app.post("/login")
async def login(data: LoginData):
    if data.username == "user1" and data.password == "pass123":
        return {"message": "Login successful"}
    else:
        return {"message": "Invalid credentials"}

client = TestClient(app)

def test_login_success():
    response = client.post("/login", json={"username": "user1", "password": "pass123"})
    # Your code here

Need a hint?

Use assert response.status_code == 200 to check the status code.