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

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

{% block main %}
<div class="box" >
    <div class="jar" ><h2>欢迎登陆</h2></div>
    <form action="{{ url_for("login") }}" method="post">
        <div class="input_box">
            账号　     <input id="name" name="username" type="text" placeholder="请输入用户名"></div><br>
        <div class="input_box">
            密码　     <input id="password" name="password" type="password" placeholder="请输入密码"></div><br>

        <div id="error_box"><br></div>
         <div class="ja">
            <button onclick="return myLogin()" >登陆</button></div>
    </form></div>
{% endblock %}

function fnLogin() {
        var uname = document.getElementById("name")
        var uError = document.getElementById("error_box")
        var upassword = document.getElementById("password")
        var upasswordagain = document.getElementById("passwordagain")
        var isError =true;
        uError.innerHTML="<br>"
         if(uname.value.length<6 || uname.value.length >20){
        uError.innerHTML="用户名应为6到20字符";
        isError = false;
         return isError;
    }else if ((uname.value.charCodeAt(0)>=48) && (uname.value.charCodeAt(0)<=57)){
        uError.innerHTML="第一位只能是字母";
        isError = false;
         return isError;
    }
    if(upassword.value.length<6 || upassword.value.length>12){
        uError.innerHTML="密码应为6到20字符";
        isError = false;
         return isError;
    }
    return isError;
    window.alert("登录成功")
}

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)

db = SQLAlchemy(app)
# 创建用户模型
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)

# db.create_all()

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

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


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


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

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

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

SECRET_KEY=os.urandom(24)