完成登录功能，用session记住用户名

登录功能完成：

1. js:设置return
2. html:设置
   1. form
   2. input

   3. onclick="return fnLogin()"

3. py:
   1. @app.route设置methods
   2. GET
   3. POST
      1. 读取表单数据
      2. 查询数据库
         1. 用户名密码对：
            1. 记住用户名
            2. 跳转到首页
         2. 用户名密码不对：
            1. 提示相应错误。

session：

1. 从`flask`中导入`session`
2. 设置`SECRET_KEY`
3. 操作字典一样操作`session`：增加用户名`session['username']=`username

#login.html

{% extends 'base.html' %}
{% block title %}欢迎您的登录{% endblock %}

{% block head %}
    <link href="{{ url_for('static',filename='css/js31.css') }}" rel="stylesheet" type="text/css">
    <script src="{{ url_for('static',filename='js/login.js') }}"></script>
{% endblock %}

{% block main %}
    <div class="box">
            <h2>登录页面</h2>
            <form action="{{ url_for('login') }}" method="post">
            <div class="input_box">
               用户名： <input id="uname" type="text" placeholder="请输入用户名">
        </div>
             <div class="input_box">
                密码： <input id="upass" type="password" placeholder="请输入密码">
             </div>
        <div id="error_box"><br></div>
        <div class="input_box">
            <button onclick="return fnLogin()">登录</button>
            <button onclick=window.alert("此页面询问您是否要离开：您输入的数据可能不会被保存")>取消</button></div>
            </form>
    </div>
{% endblock %}

　　

from flask import Flask,render_template,request,redirect,url_for,session
from flask_sqlalchemy import SQLAlchemy

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

#db.create_all()


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

@app.route('/login/',methods=['GET','POST'])
def login():
    if request.method=='GET':
        return render_template('login.html')
    else:
        usern = request.form.get('username')
        passw = request.form.get('password')
        user = User.query.filter(User.username == usern).first()
        if user:
            if user.password == passw:
                session['user'] = usern
                return redirect(url_for('base'))
            else:
                return u'password error'
        else:
            return u'username is not existed.'

@app.route('/regist/',methods=['GET','POST'])
def regist():
    if request.method=='GET':
        return render_template('regist.html')
    else:
        usern=request.form.get('username')
        nickn=request.form.get('nickname')
        passw=request.form.get('password')
        user=User.query.filter(User.username==usern).first()
        if user:
            return u'username existed.'
        else:
            user1=User(username=usern,password=passw,nickname=nickn)
            db.session.add(user1)
            db.session.commit()
            return  redirect(url_for('login'))

@app.context_processor
def mycontext():
    usern=session.get('user')
    if usern:
        return {'user':usern}
    else:
        return {}


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

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