old_contents: #!/usr/bin/env python
  #
  # Copyright 2007 Google Inc.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  #
  import webapp2
  
  class MainHandler(webapp2.RequestHandler):
      def get(self):
          self.response.write('Hello world!')
  
  app = webapp2.WSGIApplication([
      ('/', MainHandler)
  ], debug=True)
  
new_contents: #!/usr/bin/env python
  #
  # Copyright 2007 Google Inc.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  #
  import jinja2
  import os
  import webapp2
  
  template_dir = os.path.dirname(__file__)
  jinja_env = jinja2.Environment(
      loader = jinja2.FileSystemLoader(template_dir),
      autoescape = True)
  
  class BaseHandler(webapp2.RequestHandler):
      
      def write(self, *a, **kw):
          self.response.out.write(*a, **kw)
          
      def render_str(self, file_name, **params):
          template = jinja_env.get_template(file_name)
          return template.render(params)
          
      def render(self, file_name, **kw):
          self.write(self.render_str(file_name, **kw))
          
  
  class MainHandler(webapp2.RequestHandler):
      def get(self):
          self.response.write('Hello world!')
  
  app = webapp2.WSGIApplication([
      ('/', MainHandler)
  ], debug=True)
  
subject: create a base handler class to deal with templates