flask session – TypeError: ‘type’ object does not support item assignment

from flask import Flask, session
from flask_session import Session


SESSION_TYPE = 'redis'


app = Flask(__name__)
app.config.from_object(__name__)
Session(app)


@app.route('/set/')
def set():
    # check here
    # it is flask's session, not flask_session's Session object
    session['key'] = 'value'
    return 'ok'


@app.route('/get/')
def get():
    return session.get('key', 'not set')


if __name__ == "__main__":
    app.run(debug=True)