CORS middleware setup in FastAPI - Mini Project: Build & Apply

CORS middleware setup

📖 Scenario: You are building a simple FastAPI web application that will be accessed from different websites. To allow these websites to communicate with your API safely, you need to set up CORS (Cross-Origin Resource Sharing) middleware.

🎯 Goal: Set up CORS middleware in a FastAPI app to allow requests from specific origins.

📋 What You'll Learn

Create a FastAPI app instance

Define a list of allowed origins

Add CORS middleware to the app with the allowed origins

Configure the middleware to allow all methods and headers

💡 Why This Matters

🌍 Real World

Many web APIs need to allow requests from web pages hosted on different domains. Setting up CORS middleware correctly lets your API communicate safely with these web clients.

💼 Career

Understanding and configuring CORS is essential for backend developers working with web APIs to ensure secure and functional cross-origin requests.

1

Create a FastAPI app instance

Import FastAPI from fastapi and create a variable called app that is an instance of FastAPI().

FastAPI

# Import FastAPI and create app instance
# Your code here

Hint

Use from fastapi import FastAPI and then app = FastAPI().

2

Define allowed origins list

Create a list called origins with these exact strings: "http://localhost" and "http://localhost:3000".

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Define allowed origins list
# Your code here

Hint

Make a list named origins with the two URLs as strings.

3

Add CORS middleware to the app

Import CORSMiddleware from fastapi.middleware.cors. Then add CORS middleware to app using app.add_middleware with these arguments: allow_origins=origins, allow_credentials=True, allow_methods=["*"], and allow_headers=["*"].

FastAPI

from fastapi import FastAPI

app = FastAPI()

origins = ["http://localhost", "http://localhost:3000"]

# Import CORSMiddleware and add it to app
# Your code here

Hint

Use app.add_middleware with CORSMiddleware and set the parameters exactly as described.

4

Complete the FastAPI app with a root path

Add a root path operation to app using @app.get("/") that returns a dictionary with {"message": "Hello World"}.

FastAPI

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

origins = ["http://localhost", "http://localhost:3000"]

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"]
)

# Add a root path that returns {"message": "Hello World"}
# Your code here

Hint

Use @app.get("/") decorator and define an async function root that returns the dictionary.

Practice

1. What is the main purpose of adding CORS middleware in a FastAPI application?

easy

A. To speed up the API response time

B. To control which external websites can access your API

C. To handle database connections securely

D. To log all incoming requests for debugging

Solution

1. Step 1: Understand CORS middleware role

   CORS middleware is used to manage cross-origin requests, which means controlling which websites can call your API.

2. Step 2: Identify the correct purpose

   Among the options, only controlling external website access matches the role of CORS middleware.

3. Final Answer:

   To control which external websites can access your API -> Option B

4. Quick Check:

   CORS controls access permissions [OK]

Hint: Remember: CORS = Cross-Origin Resource Sharing control [OK]

Common Mistakes:

• Confusing CORS with performance optimization
• Thinking CORS manages database security
• Assuming CORS logs requests

2. Which of the following is the correct way to add CORS middleware in FastAPI?

easy

A. app.middleware(CORSMiddleware, allow_origins=["*"])

B. app.use(CORSMiddleware, allow_origins=["*"])

C. app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["GET"])

D. app.add_cors(allow_origins=["*"])

Solution

1. Step 1: Recall FastAPI middleware syntax

   FastAPI uses app.add_middleware() to add middleware components like CORSMiddleware.

2. Step 2: Check option syntax correctness

   app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["GET"]) uses app.add_middleware with CORSMiddleware and proper parameters, matching FastAPI docs.

3. Final Answer:

   app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["GET"]) -> Option C

4. Quick Check:

   Use add_middleware() to add CORS [OK]

Hint: FastAPI middleware always uses add_middleware() method [OK]

Common Mistakes:

• Using app.use() which is not FastAPI syntax
• Trying app.middleware() instead of add_middleware()
• Calling a non-existent add_cors() method

3. Given this FastAPI code snippet, what will be the effect of the CORS middleware?

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://example.com"],
    allow_methods=["GET", "POST"],
    allow_headers=["*"],
)

@app.get("/")
async def root():
    return {"message": "Hello"}

medium

A. Only requests from https://example.com with GET or POST methods are allowed

B. All origins and methods are allowed

C. No requests are allowed because allow_origins is too restrictive

D. Only GET requests from any origin are allowed

Solution

1. Step 1: Analyze allow_origins and allow_methods

   allow_origins is set to ["https://example.com"], so only that origin is allowed. allow_methods includes GET and POST.

2. Step 2: Determine request permissions

   Requests from other origins or methods not in GET/POST will be blocked by CORS policy.

3. Final Answer:

   Only requests from https://example.com with GET or POST methods are allowed -> Option A

4. Quick Check:

   allow_origins and allow_methods restrict access [OK]

Hint: Check allow_origins and allow_methods lists carefully [OK]

Common Mistakes:

• Assuming allow_origins=["*"] when it is not
• Ignoring allow_methods restrictions
• Thinking all origins are allowed by default

4. Identify the error in this FastAPI CORS middleware setup:

app.add_middleware(
    CORSMiddleware,
    allow_origins="*",
    allow_methods=["GET", "POST"],
    allow_headers=["*"]
)

medium

A. CORSMiddleware must be imported from fastapi.middleware.security

B. allow_methods should be a string, not a list

C. allow_headers cannot contain '*'

D. allow_origins should be a list, not a string

Solution

1. Step 1: Check allow_origins type

   allow_origins must be a list of strings, but here it is a single string "*".

2. Step 2: Verify other parameters

   allow_methods is correctly a list, allow_headers can accept ["*"] as a list.

3. Final Answer:

   allow_origins should be a list, not a string -> Option D

4. Quick Check:

   allow_origins requires a list [OK]

Hint: Always use a list for allow_origins, even if one item [OK]

Common Mistakes:

• Passing allow_origins as a string instead of list
• Misunderstanding allow_methods type
• Wrong import path for CORSMiddleware

5. You want your FastAPI backend to accept requests from two frontend domains: https://app1.example.com and https://app2.example.com. You also want to allow all HTTP methods and headers. Which CORS middleware setup is correct?

hard

A. app.add_middleware(CORSMiddleware, allow_origins=["https://app1.example.com", "https://app2.example.com"], allow_methods=["*"], allow_headers=["*"])

B. app.add_middleware(CORSMiddleware, allow_origins=["*"])

C. app.add_middleware(CORSMiddleware, allow_origins=["https://app1.example.com", "https://app2.example.com"], allow_methods=["GET", "POST"], allow_headers=["Content-Type"])

D. app.add_middleware(CORSMiddleware, allow_origins="https://app1.example.com,https://app2.example.com", allow_methods=["*"], allow_headers=["*"])

Solution

1. Step 1: Set allow_origins correctly

   To allow two specific domains, use a list with both URLs as strings.

2. Step 2: Allow all methods and headers

   Using ["*"] for allow_methods and allow_headers allows all HTTP methods and headers.

3. Step 3: Check for syntax correctness

   app.add_middleware(CORSMiddleware, allow_origins=["https://app1.example.com", "https://app2.example.com"], allow_methods=["*"], allow_headers=["*"]) correctly uses a list for origins and lists with "*" for methods and headers.

4. Final Answer:

   app.add_middleware(CORSMiddleware, allow_origins=["https://app1.example.com", "https://app2.example.com"], allow_methods=["*"], allow_headers=["*"]) -> Option A

5. Quick Check:

   List origins + wildcard methods/headers [OK]

Hint: Use list for origins and ["*"] to allow all methods/headers [OK]

Common Mistakes:

• Passing origins as a single comma string
• Using allow_methods with limited verbs instead of wildcard
• Setting allow_origins to ["*"] when only specific domains needed