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__='users'
       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()
   
   #查
   #user = User.query.filter(User.username == 'sk').first()
   #print(user.id,user.password)
   
   #改
   #user = User.query.filter(User.username == 'mis1234').first()
   #user.password='888888'
   #user.username='sk'
   #db.session.commit()
   #print(user.id,user.password)
   
   #删
   #user = User.query.filter(User.username=='sk').first()
   #db.session.delete(user)
   #db.session.commit()
   
   
   #增
   #user = User(username='mis1234',password='111111')
   #db.session.add(user)
   #db.session.commit()
   
   @app.route('/')
   def index():
       return  render_template('base.html')
   
   
   @app.route('/register/',methods=['GET','POST'])
   def register():
       if request.method=='GET':
           return render_template('register.html')
       else:
           usern = request.form.get('username')
           nickn = request.form.get('nickname')
           passw = request.form.get('password')
           user = User.query.filter(User.username == usern).first()
           if user:
               return 'username existed.'
           else:
               user1 = User(username = usern,password = passw,nickname = nickn)
               db.session.add(user1)
               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:
           usern = request.form.get('username')
           passw = request.form.get('password')
           user = User.query.filter(User.username == usern).first()
           if user:
               if user.password==passw:
                   session['user']=usern
                   return redirect(url_for('index'))
               else:
                   return 'password error.'
           else:
               return 'username is not existed.'
   
   @app.route('/question')
   def question():
       return render_template('question.html')
   
   
   if __name__ == '__main__':
       app.run()

   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)

    

   function fnLogin() {
       var oUsername = document.getElementById("username");
       var oUserpass = document.getElementById("userpass");
       var oError = document.getElementById("error_box");
       oError.innerHTML = "<br>";
       if (oUsername.value.length > 20 || oUsername.value.length < 6) {
           oError.innerHTML = "用户名6到20位";
           return false;
       }
       else if ((oUsername.value.charCodeAt(0) >= 48) && oUsername.value.charCodeAt(0) <= 57) {
           oError.innerHTML = "first number";
           return false;
       }
       else for (var i = 0; i < oUsername.value.length; i++) {
               if ((oUsername.value.charCodeAt(i) < 48 || oUsername.value.charCodeAt(i) > 57) && (oUsername.value.charCodeAt(i) < 97 || oUsername.value.charCodeAt(i) > 122)) {
                   oError.innerHTML = "only letter or name.";
                   return false;
               }
           }
       if (oUserpass.value.length > 20 || oUserpass.value.length < 6) {
           oError.innerHTML = "密码6到20位";
           return false;
       }
       else if ((oUserpass.value.charCodeAt(0) >= 48) && oUserpass.value.charCodeAt(0) <= 57){
           oError.innerHTML = "first number";
           return false;
       }
       return true
       //window.alert("登录成功！")
   }

   {% extends'base.html' %}
   
   {% block title %}
   登录
   {% endblock %}
   {% block head %}
       <link rel="stylesheet" type="text/css" href="{{ url_for('static',filename='css/js23.css') }}">
       <script src="{{ url_for('static',filename='js/js23.js') }}"></script>
   {% endblock %}
   {% block main %}
   <div class="bg-img">
      <div class="box">
          <form action="{{ url_for('login') }}" method="post">
          <h1>登录</h1>
          <div class="input_box">
              <h3>username</h3>
              <input id="username" type="text" placeholder="请输入用户名" name="username">
          </div>
          <div class="input_box">
              <h3>userpass</h3>
   
              <input id="userpass" type="password" placeholder="请输入密码" name="password">
          </div>
          <br>
          <div class="link_box">
              <a href="{{ url_for('register') }}">
                  没有账号，立即注册
                  <b> ></b>
              </a>
          </div>
          <div id="error_box"><br></div>
          <div class="input_box">
              <button onclick="return fnLogin()">LOGIN</button>
              <button onclick=window.alert(fnLogin())>RESET</button>
          </div>
              </form>
      </div>
   </div>
   {% endblock %}