i often get this error The browser (or proxy) sent a request that this server could not understand
<p>I am very new to python and just trying very simple code 
this is my code</p>

<p>-------------------proapp.py</p>

<pre><code>from flask import Flask, render_template, url_for, request, flash, redirect
from flaskext.sqlalchemy import SQLAlchemy
import types

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
app.config['DEBUG'] = True
db = SQLAlchemy(app)

class Basics(db.Model):

    __tablename__ = 'basics'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)

    def __init__(self, username):
          self.username = username 

    def __repr__(self):
        return '&lt;User %r&gt;' % self.username

@app.route('/')

def mainlists():
          return render_template('index.html', lists=Basics.query.all())

@app.route('/', methods=['GET', 'POST'])

def createlist():
      if request.method == 'POST':
           new_list = Basics(request.form['lists'])
           db.session.add(new_list)
           db.session.commit()
           flash(u' item was successfully created')
           return redirect(url_for('mainlists'))

if __name__ == '__main__':
    app.run()
</code></pre>

<p>and my HTML code is this:</p>

<pre><code>&lt;html&gt;
  &lt;head&gt;
      &lt;style&gt; 
         h1
           {
             padding: 0px 0 0 13px;
             margin: 15px 0 15px 0;
             color:black;
             height:17px;
             position:relative
            }
            h2
            {  
              height:17px;
              color:gray  
             }
       &lt;/style&gt;
    &lt;/head&gt;
       &lt;body background="swirlies.gif"&gt;
            &lt;h1&gt;&lt;i&gt;&lt;b&gt;Memelist&lt;/b&gt;&lt;/i&gt;&lt;/h1&gt;
               &lt;hr size="3" noshade color="gray"&gt;
                 &lt;marquee behavior="alternate"&gt; &lt;h4&gt;&lt;i&gt;Share your memories&lt;/i&gt;&lt;/h4&gt;&lt;/marquee&gt;     
                       &lt;ul&gt;
                          {% for item in lists %}
                          &lt;li&gt;{{ item.username }}&lt;/li&gt;
                          {% endfor %}
                      &lt;/ul&gt;

                   &lt;form action="{{ url_for('createlist') }}" method=post &gt;

                   &lt;center&gt;  Enter the list&lt;input type="text" value=""/&gt;
              &lt;input type="submit" value="submit"/&gt;&lt;/center&gt;
          &lt;/form&gt;
       &lt;/body&gt;
&lt;/html&gt;
</code></pre>

<p>Can anyone pls fix this error and make my code to execute</p>
