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<!doctype html>
<html>
   <body>

      <h1>Login</h1>

      {% if error %}
      <p><strong>Error:</strong> {{ error }}
      {% endif %}

      <form action = "" method = post>
         <dl>
            <dt>Username:</dt>

            <dd>
               <input type = text name = username
                  value = "{{request.form.username }}">
            </dd>

            <dt>Password:</dt>
            <dd><input type = password name = password></dd>
         </dl>
         <p><input type = submit value = Login></p>
      </form>

   </body>
</html>

<!doctype html>
<html>

   <head>
      <title>Flask Message flashing</title>
   </head>
   <body>

      {% with messages = get_flashed_messages() %}
         {% if messages %}
            <ul>
               {% for message in messages %}
               <li<{{ message }}</li>
               {% endfor %}
            </ul>
         {% endif %}
      {% endwith %}

      <h1>Flask Message Flashing Example</h1>
      <p>Do you want to <a href = "{{ url_for('login') }}">
         <b>log in?</b></a></p>

   </body>
</html>

from flask import Flask, flash, redirect, render_template, request, url_for
app = Flask(__name__)
app.secret_key = 'random string'

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

@app.route('/login', methods = ['GET', 'POST'])
def login():
   error = None

   if request.method == 'POST':
      if request.form['username'] != 'admin' or
         request.form['password'] != 'admin':
         error = 'Invalid username or password. Please try again!'
      else:
         flash('You were successfully logged in')
         return redirect(url_for('index'))

   return render_template('login.html', error = error)

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