Minimal Flask server for Google API credentials OAuth2 authentication.

auth_server.py

import os
from flask import Flask, session, abort, redirect, request
from google_auth_oauthlib.flow import Flow


app = Flask(__name__)
app.secret_key = os.urandom(24)

# Get client secrets file from https://console.cloud.google.com/apis/credentials -> OAuth client ID -> Web application
flow = Flow.from_client_secrets_file(
    client_secrets_file="<client-secrets>.json",
    scopes=["https://mail.google.com/"],
    redirect_uri="https://<auth-server-url>/callback"
)

@app.route("/")
def login():
    authorization_url, state = flow.authorization_url()
    session["state"] = state
    return redirect(authorization_url)

@app.route("/callback")
def callback():
    flow.fetch_token(authorization_response=request.url)
    if not session["state"] == request.args["state"]:
        abort(500)
    return redirect("<https://<app-url>")