OAuth2 overview in Flask - Step-by-Step Execution

Concept Flow - OAuth2 overview

User wants to access app

↓

App redirects user to OAuth2 Provider

↓

User logs in and grants permission

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Provider sends authorization code to app

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App exchanges code for access token

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App uses token to access user data

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User data received and app shows content

This flow shows how a user logs in via OAuth2: the app redirects to a provider, user authorizes, app gets a token, then accesses user data.

Execution Sample

Flask

from flask import Flask, redirect, request
app = Flask(__name__)

oauth_provider_url = 'https://provider.com/oauth2/authorize'

@app.route('/login')
def login():
  return redirect(oauth_provider_url)

@app.route('/callback')
def callback():
  code = request.args.get('code')
  token = exchange_code_for_token(code)
  user_info = get_user_info(token)
  return f"Hello {user_info['name']}"

This Flask code redirects user to OAuth2 provider, handles callback with code, exchanges it for token, then fetches user info.

Execution Table

Step	Action	Input/Condition	Output/Result
1	User visits /login	User clicks login	Redirect to OAuth2 provider URL
2	User logs in at provider	User enters credentials	Provider authenticates user
3	Provider redirects to /callback	Authorization code sent in URL	App receives code
4	App exchanges code for token	Code received	Access token received
5	App requests user info	Access token used	User info JSON received
6	App displays user info	User info received	Page shows 'Hello {user name}'
7	End	No more steps	OAuth2 login flow complete

💡 OAuth2 flow ends after user info is displayed and user is logged in.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
code	None	Received from URL	Used for token exchange	Used for user info request	None (used up)
token	None	None	Received from provider	Used to get user info	Valid access token
user_info	None	None	None	Received JSON data	User info dict with name

Key Moments - 3 Insights

Why does the app redirect the user to the OAuth2 provider instead of logging in directly?

The app redirects because OAuth2 delegates login to the provider, so the app never handles user passwords directly. See execution_table step 1 and 2.

What is the authorization code and why is it important?

The code is a temporary token sent by the provider to the app after user login. The app exchanges it for an access token. See execution_table step 3 and 4.

Why does the app need an access token?

The access token lets the app request user data securely from the provider. Without it, the app can't get user info. See execution_table step 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what does the app receive at step 4?

A Access token

B Authorization code

C User info JSON

D Redirect URL

💡 Hint

Check the 'Output/Result' column for step 4 in execution_table.

At which step does the user grant permission to the app?

A Step 3

B Step 1

C Step 2

D Step 5

💡 Hint

Look at the 'Action' and 'Input/Condition' columns around user login and authorization.

If the app never receives the authorization code, what happens?

A App can still get user info

B App cannot get access token

C User is logged in automatically

D App redirects user again

💡 Hint

Refer to variable_tracker for 'code' and execution_table step 4.

Concept Snapshot

OAuth2 lets apps log users in via a provider.
User is redirected to provider to login and authorize.
Provider sends an authorization code to app.
App exchanges code for access token.
App uses token to get user info.
This keeps user passwords safe and secure.

Full Transcript

OAuth2 is a way for apps to let users log in using another service, like Google or Facebook. The app sends the user to the OAuth2 provider to log in and approve access. After approval, the provider sends a code back to the app. The app then exchanges this code for an access token. Using this token, the app can request user information securely. This process protects user passwords and lets apps access only allowed data.