1.  用上下文处理器app_context_processor定义函数
   1. 获取session中保存的值
   2. 返回字典
2. 在父模板中更新导航，插入登录状态判断代码。
   1. 注意用{% ... %}表示指令。
   2. {
      { }}表示变量
3. 完成注销功能。
   1. 清除session
   2. 跳转
          
                      
                        
                {% block head %}{% endblock %}
               
          
                
                
          
                首页    
                下载    
                    
                搜索    {%  if name %}        
                
      {
             { name }}        
                
      注销    {% else %}         
                
      登陆         
                
      注册     {% endif %}  
                    
               
      {% block main %}{% endblock %}
      from flask import Flask,request,render_template,redirect,url_for,sessionfrom flask_sqlalchemy import SQLAlchemyimport configfrom functools import wrapsfrom  datetime import datetimeapp = 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(50))#db.create_all()#增加数据#user=User(username = 'shanshan1',password = '111111')#db.session.add(user)#db.session.commit()#查询数据#user=User.query.filter(user.username == 'shanshan1').first()#print(user.id,user.username,user.password)#修改数据#user=User.query.filter(user.username == 'shanshan3').first()#user.password = '0000'#db.session.commit()#删除数据#user=User.query.filter(user.username == 'shanshan3').first()#db.session.delete(user)#db.session.commit()@app.route('/')def index():    return  render_template('shouye.html')@app.route(   '/manhua')def manhua():    return  render_template('manhua.html')@app.route('/login',methods=['GET','POST'])def login():    if request.method =='GET':        return render_template('denglu.html')    else:        name = request.form.get('name')        passw = request.form.get('password')        user = User.query.filter(User.username == name).first()    if user:       if user.password==passw:           session['user']=name           return redirect(url_for('manhua'))       else:           return "密码错误！"    else:        return "用户名不存在！"@app.context_processordef mycontext():    usern=session.get('user')    if usern:        return {
             'name':usern}    else:        return {}@app.route('/logout')def logout():    session.clear()    return redirect(url_for('manhua'))@app.route("/register",methods=['GET','POST'])def register():    if request.method == 'GET':        return render_template("zhuce.html")    else:        name = request.form.get('name')        passw = request.form.get('password')        user = User.query.filter(User.username == name).first()    if user:        return "用户名已存在！"    else:        user = User(username=name, password=passw)        db.session.add(user)        db.session.commit()        return render_template("denglu.html")if __name__ == '__main__':    app.run(debug=True)