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
   function myLogin() {        var oUname = document.getElementById("uname");        var oPass = document.getElementById("upass");        var oError = document.getElementById("error_box");        oError.innerHTML = "
   ";        if (oUname.value.length < 6 || oUname.value.length > 20) {            oError.innerHTML = "用户名为6-20位。";            return false;        }        else if((oUname.value.charCodeAt(0)>=48)&&(oUname.value.charCodeAt(0)<=57)){            oError.innerHTML="首位不能是数字";            return false;        }        else  for(var i=0;i
          
           57)&&(oUname.value.charCodeAt(i)<97||oUname.value.charCodeAt(i)>122)){                oError.innerHTML="用户名只能包括字母和数字";                return false;                }            }        if (oPass.value.length < 6 || oPass.value.length > 20) {            oError.innerHTML = "密码为6-20位。";            return false;        }    return true;        // window.alert("登录成功！");    }
          
   {% extends 'base.html'%}{% block title %}登录{% endblock %}{% block head %}    
          {% endblock %}{% block main %}
          
           
           
   登录
       
           
            
            
                          username:
                      
            
            
            
                          password:
                      
            
            
            
             
            
            
            
                
             Login          
            
               
           
          
   {% endblock %}
   from flask import Flask,redirect,request,url_for,sessionfrom flask import render_templatefrom flask_sqlalchemy import SQLAlchemyimport configapp = Flask(__name__)app.config.from_object(config)db=SQLAlchemy(app)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)    nickname=db.Column(db.String(20))# db.create_all()#查询# user=User.query.filter(User.username=='mis1234').first()# print(user.id,user.username,user.password)#增加# user=User(username='mis1234',password='111111',nickname='l')# db.session.add(user)# db.session.commit()#增加# user=User(username='luk',password='123456',nickname='lu')# db.session.add(user)# db.session.commit()# 修改# user=User.query.filter(User.username=='mis1234').first()# user.password='12345678'# db.session.commit()## # 删除# user=User.query.filter(User.username=='mis1234').first()# db.session.delete(user)# db.session.commit()@app.route('/')def base():    return render_template('base.html')@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('shouye'))            else:                 return '密码错误。'        else:            return  '用户名不存在。'@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 '用户名已存在。'        else:            user1=User(username=usern,password=passw,nickname=nickn)            db.session.add(user1)            db.session.commit()            return redirect(url_for('login'))#重定回向登录页面@app.route('/question/')def question():    return render_template('question.html')@app.route('/shouye/')def shouye():    return render_template('shouye.html')if __name__ == '__main__':    app.run(debug=True)
   import osSQLALCHEMY_DATABASE_URI='mysql+pymysql://root:@127.0.0.1:3306/mis_db?charset=utf8'SQLALCHEMY_TRACK_MODIFICATIONS=FalseSECRET_KEY=os.urandom(24)