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):
       __table__name = '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(50))
   
   db.create_all()
   
   
   @app.route('/')
   def index():
       return render_template('index.html')
   
   @app.route('/regist/',methods=['GET','POST'])
   def regist():
       if request.method=='GET':
           return render_template('regist.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 u'username existed'
           else:
               user=User (username=username,password=password,nickname=nickname)
               db.session.add(user)
               db.session.commit()
               return redirect(url_for('login'))
   
   
   
   @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('index'))
               else:
                   return u'password error'
           else:
               return u'username is not existed'
   
   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)

   login.html

   {% extends'base.html' %}
   {% block title %}
       登录
   {% endblock %}
   {% block head %}
       <script src="{{ url_for('static',filename='js/login.js') }}"></script>
       <link rel="stylesheet" href="{{ url_for('static',filename='css/login.css')}}" type="text/css">
   {% endblock %}
   
   {% block main %}
       <div class="bigbox">
         <div class="box">
             <h2>登录</h2>
           <form action="{{ url_for('login') }}"method="post">
          <div class="input_box">
              账户：<input id="umane"type="text"placeholder="请输入用户名"name="username">
          </div>
           <div class="input_box">
              密码：<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="myLogin()">登录</button>
                  <button onclick=window.alert("是否取消登录？")>取消</button></div>
             </div>
                  </div>
   
   {% endblock %}

   login.js

    function myLogin() {
               var oUname = document.getElementById("umane");
               var oError = document.getElementById("error_box");
                var oUpass = document.getElementById("upass");
                 var isError = true;
               oError.innerHTML = "<br>"
        //oUname
           if (oUname.value.length>12 || oUname.value.length<6){
               oError.innerHTML = "name:6-12";
                isError=false;
                return isError;
            } else if((oUname.value.charCodeAt(0)>=48) && oUname.value.charCodeAt(0)<=57){
   
               oError.innerHTML = "首字母不能是数字";
                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 = "只能包含字母和数字";
                        isError=false;
                         return isError;
                   }
               }
   
   
               if(upass.value.length>12 || upass.value.length<6){
                   oError.innerHTML="password;6-12";
                    isError=false;
                   return isError;
               }
               window.alert("登陆成功 ！")
                return isError
   }