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
      from flask import Flask,render_template,request,redirect,url_for,sessionfrom 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()@app.route('/')def vi():    return render_template("base.html")@app.route('/first')def first():    return render_template("first.html")@app.route("/login",methods=["GET","POST"])def login():    if request.method == "GET":      return render_template("login.html")    else:        usern = request.form.get("usern")        passw = request.form.get("userp")        user = User.query.filter(User.username==usern).first()        if user:            if user.password == passw:                session["user"] = usern                # session.permanent = True                return redirect(url_for("first"))            else:                return "密码错误，请重新输入！"        else:            return "用户名不存在！"@app.route("/regist",methods=["GET","POST"])def regist():    if request.method == "GET":        return render_template("regist.html")    else:        usern1 = request.form.get("newn")        passw1 = request.form.get("newp")        user = User.query.filter(User.username==usern1).first()        if user:            return "用户名已存在！"        else:            user = User(username = usern1,password = passw1)            db.session.add(user)            db.session.commit()            return redirect(url_for("login"))@app.context_processordef context():    usern=session.get("user")    if usern:        return{
             'username':usern}    else:        return {}@app.route('/clear')def clear():    session.clear()    return redirect(url_for('first'))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)
          
                       
                   {% block head %}{% endblock %}
               
          
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                搜索    {% if username %}        
                {
             { username }}        
                注销    {% else %}    
                登录    
                注册    {% endif %}    
                
               
      {% block main %}{% endblock %}
               
          
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