完成登录功能，用session记住用户名

from flask import Flask, render_template,request,redirect,url_for,session
from flask_sqlalchemy import SQLAlchemy
# import config
from functools import wraps
from datetime import datetime

app = Flask(__name__)

db = SQLAlchemy(app)

class User(db.Model):
    __tablename__ = 'user'
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    username = db.Column(db.String(20), nullable=False)
    password = db.Column(db.String(20), nullable=False)
    nickname = db.Column(db.String(20))

@app.route('/')
def index():
    return render_template('test.html')


@app.route('/login',methods=['GET','POST'])
def login():
    if request.method == 'GET':
        return render_template('test.html')
    else:
        username = request.form.get('username')
        password = request.form.get('password')
        nickname = request.form.get('nickname')
        user = User.query.filter(User.username == username).first()
        if user:
            return '用户名已存在'
        else:
            user = User(username=username, password=password)
            db.session.add(user)
            db.session.commit()
            return render_template(url_for('login'))

@app.route('/regist',methods=['GET','POST'])
def regist():
    if request.method == 'GET':
        return render_template('注册.html')
    else:
        username = request.form.get('username')
        password = request.form.get('password')
        nickname = request.form.get('nickname')
        user = User.query.filter(User.username == username).first()
        if user:
            return '用户名存在'
        else:
            user = User(username = username,password = password,nickname=nickname)
            db.session.add(user)
            db.session.commit()
            return render_template(url_for('login'))

@app.route('/fabu')
def fabu():
    return render_template('fabu.html')

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