登录功能完成：

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

   3. οnclick="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
   import osDEBUG = TrueSECRET_KEY = os.urandom(24)DIALECT = 'mysql'DRIVER = 'mysqldb'USERNAME = 'root'PASSWORD = 'ROOT'HOST = '127.0.0.1'DATABASE = 'mytest'SQLALCHEMY_DATABASE_URI = 'mysql+pymysql://root:123456@127.0.0.1:3306/mytest?charset=utf8'SQLALCHEMY_TRACK_MODIFICATIONS = False
       {% extends 'test1.html' %}    
               {% block head %}
               {% endblock %}{% block body %}    
           
           
            
   登录
           
            
                 
             
               
                      
             
           
             
               
                      
             
           
             
              
           
             
           
             
               
              登录        
             
           
            
       
           
   {% endblock %}
   function fnLogin() {    var oUname = document.getElementById("name");    var oUpass = document.getElementById("pass");    var oError = document.getElementById("error_box");    var isError = true;    oError.innerHTML = '
   '    //    if (oUname.value.length > 20 || oUname.value.length < 6) {        oError.innerHTML = "name:6-20位";        isError = false;        return isError;    } else if ((oUname.value.charCodeAt(0) >= 48) && (oUname.value.charCodeAt(0) <= 57)) {        oError.innerHTML = "first letter.";        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(0) < 97) || (oUname.value.charCodeAt(0) > 122)) {            oError.innerHTML = "only letter or number";            isError = false;            return isError;        }    }    if (oUpass.value.length > 20 || oUpass.value.length < 6) {        oError.innerHTML = "password:6-20";        isError = false;        return isError;    }    return isError;    window.alert("登录成功！")}