session_init.py

import flask
import firebase_admin
from firebase_admin import auth
from firebase_admin import credentials

cred = credentials.Certificate('serviceAccount.json')
firebase_admin.initialize_app(cred)
app = flask.Flask(__name__)

@app.route('/sessionInit', methods=['POST'])
def session_initializer():
    id_token = flask.request.form['idToken']
    expires_in = datetime.timedelta(days=5)
    try:
        session_cookie = auth.create_session_cookie(id_token, expires_in=expires_in)
        response = flask.jsonify({'status': 'success'})
        expires = datetime.datetime.now() + expires_in
        response.set_cookie(
            'session', session_cookie, expires=expires, httponly=True)
        return response
    except auth.AuthError:
        return flask.abort(401, 'Failed to create a session cookie')