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. 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

function fndenglu() {
    var oUname = document.getElementById("uname");
    var oUpass = document.getElementById("upass");
    var oError = document.getElementById("error_box");
    var isError = true;
    oError.innerHTML = "<br>";
    // oUname
    if (oUname.value.length < 6 || oUname.value.length > 20) {
        oError.innerHTML = "用户名要6-20之位";
        isError = false;
        return isError;
    } else if ((oUname.value.charCodeAt(0) >= 48) && (oUname.value.charCodeAt(0) <= 57)) {
        oError.innerHTML = "首字母不能为数字";
        isError = false;
        return isError;
    } else for (var i = 0; i < oUname.value.length; i++) {
        if ((oUname.value.charCodeAt(i) < 48) || (oUname.value.charCodeAt(i) > 57) && ((oUname.value.charCodeAt(i) < 97)) || oUname.value.charCodeAt(i) > 122) {
            oError.innerHTML = "只能填写数字或字母";
            isNotError = false;
            return isError;
        }
    }
// oUpass
    if (oUpass.value.length > 20 || oUpass.value.length < 6) {
        oError.innerHTML = "密码要6-20位";
        isError = false;
        return isError;
    }
    return isError;
    window.alert("登录成功！")
}

{% extends 'daohang.html' %}
{% block title %}登录{% endblock %}
{% block head %}
    <link rel="stylesheet" type="text/css" href="{{ url_for('static',filename='css/denglu.css') }}">
    <script src="{{ url_for('static',filename='js/denglu.js') }}"></script>
{% endblock %}

{% block main %}
    <div class="box">

        <h2>登录</h2>
        <form action="{{ url_for('denglu') }}" method="post">
            <div class="input_box">
                <input id="uname" type="text" placeholder="请输入用户名" name="username">
            </div>
            <div class="input_box">
                <input id="upass" type="password" placeholder="请输入密码" name="password">
            </div>
            <div id="error_box"><br></div>
            <div class="input_box">
                <button onclick="fndenglu()">登录</button>
            </div>
        </form>
    </div>
{% endblock %}

@app.route('/denglu/', methods=['GET', 'POST'])
def denglu():
    if request.method == 'GET':
        return render_template('denglu.html')
    else:
        username = request.form.get('username')
        password = request.form.get('password')
        user = User.query.filter(User.username == username,User.password==password).first()
        if user:
            session['user']=username
            session.permanent=True
            return redirect(url_for('daohang'))
        else:
            return 'error username or password'

import os

DEBUG=True

SECRET_KEY=os.urandom(24)

DIALECT='mysql'
DRIVER='mysqldb'
USERNAME='root'
PASSWORD=''
HOST='127.0.0.1'
PORT='3306'
DATABASE='test'


SQLALCHEMY_DATABASE_URI = 'mysql+pymysql://root:@127.0.0.1:3306/test?charset=utf8'
SQLALCHEMY_TRACK_MODIFICATIONS = False