I'm learning Google App Engine right now with a book 'Using Google App Engine by Charles Severance'.

I am on chapter 6 and so far I have made `app.yaml`, `index.py`, `index.html` in templates folder and the CSS file in the static folder.

My `index.py` looks like this:

```
import os
import wsgiref.appengine.ext import webapp
from google.appengine.ext import webapp
from google.appengine.ext.webapp import template

class MainHandler(webapp.RequestHandler):
      def get(self):
          path=self.request.path
          temp=os.path.join(os.path.dirname(__file__, 'templates' + path)
          if not os.path.isfile(temp):
             temp=os.path.join(os.path.dirname(__file__, 'templates/index.html')

          self.response.out.write(template.render(temp,{}))

def main():
    application = webapp.WSGIApplication([('/.*', MainHandler)], debug=True)
    wsgiref.handlers.CGIHandler().run(application)

if __name == '__main__':
   main()

```

Why am I am getting this error?

```
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/wsgiref/handlers.py", line 86, in run
    self.finish_response()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/wsgiref/handlers.py", line 127, in finish_response
    self.write(data)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/wsgiref/handlers.py", line 202, in write
    **assert type(data) is StringType,"write() argument must be string"**

AssertionError: write() argument must be string

```