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

   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,autioncrement=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('index.html')
   
   
   
   
   @app.route("/zhuce/", 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).first()
           if user:
               session['user'] = username
               session.permanent = True
               return redirect(url_for('index'))
           else:
               return 'error username or password'
   
   @app.route("/zhuce/", methods=['GET', 'POST'])
   def zhuce():
       if request.method == 'GET':
           return render_template('zhuce.html')
       else:
           username = request.form.get('username')
           password = request.form.get('password')
           user = User.query.filter(User.username == username).first()
           if user:
               return 'username exited'
           else:
               user = User(username=username, password=password)
               db.session.add(user)
               db.session.commit()
               return redirect(url_for('denglu'))
   
   @app.route("/question/")
   def question():
       return render_template('question.html')
   
   if __name__ == '__main__':
       app.run(debug=True)

   function myLogin() {
                var oUname = document.getElementById("uname");
                var oError = document.getElementById("error_box");
                var oUpass=document.getElementById("upass");
                oError.innerHTML="<br>";
                //uname
                if (oUname.value.length<6 || oUname.value.length>20){
                     oError.innerHTML="用户名6-20位";
                     return;
                    }else if((oUname.value.charCodeAt(0)>48) && (oUname.value.charCodeAt(0)<=57)){
                         oError.innerHTML = "first number";
                         return;
                }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 = "only letter or number";
                         return;
                     }
                }
                //upass
                if(oUpass.value.length<6||oUpass.value.length>20){
                     oError.innerHTML="密码6-20位"
                     return;
                }
                if((oUname.value.length<6||oUname.value.length>20)&&(oUpass.value.length<6||oUpass.value.length>20)){
                    oError.innerHTML="用户名密码要求6-20位"
                    return;
                }
                   window.alert("登陆成功！")
                }

    

   {% extends'base.html' %}
   {% block title %}
       登陆
   {% endblock %}
   {% block head %}
       <script src="{{ url_for('static',filename='js/denglu.js') }}"></script>
       <link rel="stylesheet" href="{{ url_for('static',filename='css/denglu.css')}}">
   {% endblock %}
   {% block main %}
   
     <div class="denglu">
   
       <h1>denglu<h1>
       <div class="form" method="post" action="{{ url_for('denglu') }}"method="post">
         <div class="input_box">
           <input id="uname" type="text" name="username" placeholder="请输入用户名" >
   
         </div>
         <div class="input_box">
           <input id="upass" type="password" name="password" placeholder="请输入密码" >
   
         </div>
         <div id="error_box"><br></div>
         <div class="input_box">
             <button onclick"return fnLogin()">登陆</div></button>
       </div>
       </form>
     </div>
   
   {% endblock %}

    

   import os
   SECRET_KEY = os.urandom(24)
   
   SQLALCHEMY_DATABASE_URI = 'mysql+pymysql://root:@localhost:3306/mis_db?charset=utf8'
   SQLALCHEMY_TRACK_MODIFICATIONS = False