FlaskでGoogle OAuthを実装する方法

pip install authlib

class User(db.Model, UserMixin):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(150), unique=True, nullable=False)
    email = db.Column(db.String(150), unique=True, nullable=False)
    password_hash = db.Column(db.String(255), nullable=False)

    @property
    def password(self):
        raise AttributeError('password is not a readable attribute')

    @password.setter
    def password(self, password):
        self.password_hash = generate_password_hash(password)

    def verify_password(self, password):
        return check_password_hash(self.password_hash, password)

from authlib.integrations.flask_client import OAuth
from authlib.integrations.base_client.errors import OAuthError

app.config['GOOGLE_CLIENT_ID'] = [クライアントID]
app.config['GOOGLE_CLIENT_SECRET'] = [クライアントシークレット]

oauth = OAuth(app)
google = oauth.register(
    name='google',
    client_id=app.config['GOOGLE_CLIENT_ID'],
    client_secret=app.config['GOOGLE_CLIENT_SECRET'],
    client_kwargs={'scope': 'openid profile email'},
    server_metadata_url='https://accounts.google.com/.well-known/openid-configuration'
)

@app.route('/login', methods=['GET', 'POST'])
def login():
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(email=form.email.data).first()
        if user and user.verify_password(form.password.data):
            login_user(user)
            return redirect(url_for('dashboard'))
        else:
            flash(_('Login Unsuccessful. Please check email and password'), 'danger')
    return render_template('login.html', form=form)

@app.route('/login/google')
def google_login():
    redirect_uri = url_for('google_auth_callback', _external=True)
    return google.authorize_redirect(redirect_uri, prompt='select_account')

@app.route('/auth/callback')
def google_auth_callback():
    try:
        token = google.authorize_access_token()
        resp = google.get('https://www.googleapis.com/oauth2/v1/userinfo', token=token)
        user_info = resp.json()
        email = user_info['email']

        user = User.query.filter_by(email=email).first()
        if user is None:
            user = User(
                email=email, 
                username=user_info['name'],
                password_hash=generate_password_hash('default_password')  
            )
            db.session.add(user)
            db.session.commit()

        login_user(user)
        return redirect(url_for('dashboard'))
    except OAuthError as error:
        flash(f'Authentication failed: {error.description}', 'danger')
        return redirect(url_for('login'))

<a href="{{ url_for('google_login') }}">
    Googleでログイン
 </a>