old_contents: from wavvy import app
  from flask import Flask, url_for, render_template, request, session, escape
  
  
  @app.route('/hello')
  @app.route('/hello/<name>')
  def hello(name=None):
      return render_template('hello.html', name=name)
  
  
  @app.route('/')
  def index():
      if session.get('logged_in', False):
          return 'Logged in as {}'.format(escape(session['username']))
      return 'You are not logged in.'
  
  
  @app.route('/login', methods=['POST', 'GET'])
  def login():
      error = None
      if request.method == 'POST':
          session['logged_in'] = True
          session['username'] = request.form['username']
          password = escape(request.form['password'])
          return 'Validating a login! U:{} P:{}'.format(escape(session['username']), password)
      return render_template('login.html', error=error)
  
  
  @app.route('/logout')
  def logout():
      session['logged_in'] = False
      if 'username' in session:
          del session['username']
      return 'You are logged out.'
  
new_contents: from wavvy import app
  from flask import Flask, url_for, render_template, request, session, escape
  
  
  def clear_session(s):
      if 'username' in s:
          del s['username']
      s['logged_in'] = False
  
  
  @app.route('/hello')
  @app.route('/hello/<name>')
  def hello(name=None):
      return render_template('hello.html', name=name)
  
  
  @app.route('/')
  def index():
      if session.get('logged_in', False):
          return 'Logged in as {}'.format(escape(session['username']))
      return 'You are not logged in.'
  
  
  @app.route('/login', methods=['POST', 'GET'])
  def login():
      error = None
      if request.method == 'POST':
          session['logged_in'] = True
          session['username'] = request.form['username']
          password = escape(request.form['password'])
          return 'Validating a login! U:{} P:{}'.format(escape(session['username']), password)
      return render_template('login.html', error=error)
  
  
  @app.route('/logout')
  def logout():
      clear_session(session)
      return 'You are logged out.'
  
subject: Generalize the logout a bit