登录之后更新导航

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
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
完成注销功能。
1. 清除session
2. 跳转

py：

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__ = '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 index():
    return render_template('index.html')

@app.route('/denglu/',methods=['GET','POST'])
def denglu():
    if request.method == 'GET':
        return render_template('denglu.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 u'password error.'
        else:
              return u'username is not existed'
@app.route('/zhuc/',methods=['GET','POST'])
def zhuc():
    if request.method=='GET':
        return render_template('zhuc.html')
    else:
        username = request.form.get('Username')
        nickname = request.form.get('nickname')
        password = request.form.get('password')
        user=User.query.filter(User.username==username).first()
        if user:
            return u'username existed.'
        else:
            user = User(username=username,password=password,nickname=nickname)
            db.session.add(user)
            db.session.commit()
            return redirect(url_for('denglu'))
@app.context_processor
def mycontext():
    usern=session.get('user')
    if usern:
        return {'username':usern}
    else:
        return {}

@app.route('/logout/')
def logout():
        session.clear()
        return redirect(url_for('index'))

@app.route('/label/')
def label():
    return render_template('label.html')

if __name__ == '__main__':
    app.run(debug=True)

html：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>登录界面</title>
    <link rel="stylesheet" href="../static/css/base.css" type="text/css">
    <script src="../static/js/base.js"></script>
     <base target="_blank" />
{% block head %}
{% endblock %}
</head>
<body id="myBody">

<nav class="nav">
    <ul>
       <img id="me" src="../static/images/sea.png">
        <a href="{{ url_for('index') }}" style="font-size: 23px ">首页</a>
        <a href="">下载</a>
        <a href="">新闻</a>
        <a href="{{ url_for('label') }}">发布问答</a>
        <a>☀</a>
        <div>
            {% if username %}
                <a href="#" style="margin-right: -10px;color:deepskyblue">{{ username }}</a>
                <a href="{{ url_for('logout') }}" >注销</a>
            {% else %}
                <a href="{{ url_for('denglu') }}" style="margin-right: -10px">登录</a>
                <a href="{{ url_for('zhuc') }}" >注册</a>
            {% endif %}
        </div>

        <img id="on_off" onclick="mySwitch()" src="../static/images/bulbon.gif" >
        <div>
{#           <button type="submit" ></button>#}
            <input type="image" src="../static/images/search.gif" onmousemove="this.src='../static/images/search.gif'" />
           <input type="text" name="search"PLACEHOLDER="输入要搜索的内容">
        </div>

    </ul>
</nav>
{% block main %}
    <br class="br">
{% endblock %}
<div id="bottom">
    <a href="">联系我们</a>
    <a href="">加入我们</a>
    <a href="">合作伙伴</a>
  <br>
    <p>Copyright &copy; 2017. Created by <a href="#" target="_blank">ben</a></p>
</div>
</div>

</body>
</html>