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Ayoub Abidi
Ayoub Abidi

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Quick Tutorial: Implementing Google OAuth2 in FastAPI (Callback Method)

#webdev #python #fastapi #googlecloud

Hey there! 👋 Let's learn how to get user information from Google using the OAuth2 callback method in FastAPI. We'll keep it super simple and focused!

What We're Building

We'll create two endpoints:

  1. One that gives us a Google login URL
  2. Another that receives Google's callback and gets user info

That's it! No complicated stuff, just the basics to get rolling.

Prerequisites 

pip install fastapi requests
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Step 1: Set Up Your Environment Variables 

OIDC_GOOGLE_CLIENT_ID=your-client-id OIDC_GOOGLE_CLIENT_SECRET=your-client-secret OIDC_GOOGLE_REDIRECT_URI=http://localhost:8000/auth/google
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Step 2: Create Your Google Handler

Create a file called google.py:

import requests def get_user_infos_from_google_token_url(code): # Exchange the code for tokens  token_response = requests.post( "https://oauth2.googleapis.com/token", data={ "code": code, "client_id": OIDC_GOOGLE_CLIENT_ID, "client_secret": OIDC_GOOGLE_CLIENT_SECRET, "redirect_uri": OIDC_GOOGLE_REDIRECT_URI, "grant_type": "authorization_code", } ) # Get the access token  access_token = token_response.json().get("access_token") # Use the access token to get user info  user_info = requests.get( "https://www.googleapis.com/oauth2/v2/userinfo", headers={"Authorization": f"Bearer {access_token}"} ).json() return { "status": bool(user_info), "user_infos": user_info }
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Step 3: Create Your FastAPI Routes

Create your routes file:

from fastapi import APIRouter, HTTPException router = APIRouter() @router.get("/google/login") async def login_google(): # Build the Google login URL  params = { "response_type": "code", "client_id": OIDC_GOOGLE_CLIENT_ID, "redirect_uri": OIDC_GOOGLE_REDIRECT_URI, "scope": "openid email profile" } # Create the query string  query_string = "&".join(f"{key}={value}" for key, value in params.items()) # Return the full auth URL  return { "url": f"https://accounts.google.com/o/oauth2/v2/auth?{query_string}" } @router.get("/google") async def auth_google(code: str = None): if not code: raise HTTPException(status_code=400, detail="No code provided") # Get user info using the code  result = get_user_infos_from_google_token_url(code) if not result['status']: raise HTTPException(status_code=400, detail="Couldn't get user info") # Return the user info!  return result['user_infos']
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How to Use It

  1. Hit /google/login to get your Google login URL: 
response = requests.get("http://localhost:8000/auth/google/login") login_url = response.json()["url"]
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  1. Open that URL in a browser. After login, Google will redirect to your callback URL with a code.

  2. Your callback endpoint will automatically:

    • Grab the code from the URL
    • Exchange it for an access token
    • Use the token to get user info
    • Return the user info to you!

The user info you get back will look something like this:

{ "id": "1234567890", "email": "user@example.com", "verified_email": true, "name": "John Doe", "picture": "https://lh3.googleusercontent.com/..." }
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That's all there is to it! Now you can use this user info however you want - create accounts, log people in, whatever you need! 🚀

Testing It Out

The easiest way to test:

  1. Start your FastAPI server
  2. Visit /docs in your browser
  3. Try the /google/login endpoint
  4. Click the URL it gives you
  5. Watch the magic happen!

Happy coding! 🎉

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