Alright, let's break down how to automate the OAuth 2.0 credential collection for multiple users across all those Google APIs you're working with. Here's a step-by-step approach tailored to your Python project:

First, you need to set up your project in the Google Cloud Console to support all required APIs and multi-user authorization:

• Enable all target APIs: Navigate to the API Library and enable Google Contacts API, Pub/Sub API, Gmail API, and any others you need.
• Create an OAuth 2.0 Client ID: Go to Credentials > Create Credentials > OAuth client ID. Choose the app type that matches your project (e.g., "Web application" for a web-based user-facing tool, "Desktop app" if this is a client-side tool). For web apps, add your callback URL(s) (e.g., https://your-app-domain.com/oauth/callback) where Google will redirect users after they grant access.
• Set up the OAuth Consent Screen: This is critical for multi-user scenarios. Fill in the required app information, then add all the scopes your project needs. For example:
  • Gmail: https://www.googleapis.com/auth/gmail.modify (use the narrowest scope possible—e.g., readonly if you don't need to modify emails)
  • Contacts: https://www.googleapis.com/auth/contacts or contacts.readonly
  • Pub/Sub: https://www.googleapis.com/auth/pubsub
    Make sure each scope is clearly explained in the consent screen's "Scope details" section—this helps with user trust and later Google verification if needed.

You'll use Google's official Python client libraries to automate the authorization flow for each user during registration. This flow gives you a refresh token, which lets you get new access tokens without re-prompting the user.

First, install the required packages:

pip install google-auth google-auth-oauthlib google-auth-httplib2 google-api-python-client

2.1 Generate the Authorization URL for Users #

When a new user registers, generate a URL that redirects them to Google's consent screen, requesting all your required scopes. Here's a code snippet:

from google_auth_oauthlib.flow import Flow

# Define all scopes your project needs
ALL_REQUIRED_SCOPES = [
    "https://www.googleapis.com/auth/gmail.modify",
    "https://www.googleapis.com/auth/contacts",
    "https://www.googleapis.com/auth/pubsub"
]

# Load your client secrets (downloaded from Google Cloud Console)
flow = Flow.from_client_secrets_file(
    "client_secrets.json",
    scopes=ALL_REQUIRED_SCOPES
)

# Configure authorization URL:
# - access_type='offline' ensures you get a refresh token
# - include_granted_scopes='true' lets users add new scopes if they've already granted some
authorization_url, state = flow.authorization_url(
    access_type="offline",
    include_granted_scopes="true",
    prompt="consent"  # Forces the user to explicitly grant access (good for first-time registration)
)

# Redirect the user to authorization_url (in a web app, use a redirect response)

2.2 Handle the Callback and Store Credentials #

After the user grants access, Google will redirect them back to your callback URL with an authorization code. Use this code to fetch the user's tokens:

# In your callback endpoint (e.g., Flask route):
code = request.args.get("code")
flow.fetch_token(code=code)

# Get the user's credentials (includes access token, refresh token, expiry)
credentials = flow.credentials

# Serialize the credentials to store them (e.g., in a database)
credentials_dict = {
    "token": credentials.token,
    "refresh_token": credentials.refresh_token,
    "token_uri": credentials.token_uri,
    "client_id": credentials.client_id,
    "client_secret": credentials.client_secret,
    "scopes": credentials.scopes
}

# Store credentials_dict securely (encrypt it first! Never store plaintext refresh tokens)
# Associate this entry with the user's ID in your database

Later, when you need to make API calls for a user, retrieve their stored credentials and use them to authenticate:

from google.oauth2.credentials import Credentials
from googleapiclient.discovery import build

# Retrieve the encrypted credentials from your database, decrypt them
user_credentials_dict = get_credentials_from_db(user_id)

# Reconstruct the credentials object
credentials = Credentials.from_authorized_user_info(
    user_credentials_dict,
    scopes=ALL_REQUIRED_SCOPES
)

# If the access token is expired, the library will automatically refresh it using the refresh token
# Example: Call Gmail API
gmail_service = build("gmail", "v1", credentials=credentials)
messages = gmail_service.users().messages().list(userId="me").execute()

# Example: Call Contacts API
people_service = build("people", "v1", credentials=credentials)
connections = people_service.people().connections().list(
    resourceName="people/me",
    pageSize=10,
    personFields="names,emailAddresses"
).execute()

• Secure Credential Storage: Refresh tokens are highly sensitive—encrypt them before storing in your database, and restrict access to the credential storage system to only necessary services.
• Scope Minimization: Always request the narrowest scope possible for each API. For example, if you only need to read contacts, use https://www.googleapis.com/auth/contacts.readonly instead of full contacts access. This reduces user friction and improves security.
• OAuth Verification: If your app is used by external users (not just your organization), you'll need to submit it for Google's OAuth consent screen verification. This removes the "Unverified App" warning users see and is required for most production apps.
• Token Refresh: The Google client libraries handle automatic token refresh, but make sure your stored credentials include the refresh token. If a refresh token is revoked (e.g., user revokes access), your app should detect this and prompt the user to re-authorize.
• User Isolation: Ensure each user's credentials are strictly tied to their account—never reuse credentials across users.

内容的提问来源于stack exchange，提问作者Veysel Kocaman