Google app engine dev_appserver.py problems

Question

I'm just starting with google app engine and I followed the basic hello world example on google app engine.

https://developers.google.com/appengine/docs/python/gettingstartedpython27/helloworld

created both files in the helloworld folder.

I don't want to use the GUI I prefer to use the mac terminal to work with this application. I want to start this application on my local host localhost:80 through the terminal.

to run my basic helloworld application locally all I say is

$ dev_appserver.py helloworld . but I get this error.

Traceback (most recent call last):
  File "/usr/local/bin/dev_appserver.py", line 184, in <module>
    _run_file(__file__, globals())
  File "/usr/local/bin/dev_appserver.py", line 180, in _run_file
    execfile(script_path, globals_)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/devappserver2/devappserver2.py", line 727, in <module>
    main()
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/devappserver2/devappserver2.py", line 720, in main
    dev_server.start(options)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/devappserver2/devappserver2.py", line 554, in start
    options.yaml_files)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/devappserver2/application_configuration.py", line 556, in __init__
    module_configuration = ModuleConfiguration(yaml_path)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/devappserver2/application_configuration.py", line 82, in __init__
    self._yaml_path)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/devappserver2/application_configuration.py", line 271, in _parse_configuration
    with open(configuration_path) as f:
IOError: [Errno 2] No such file or directory: 'helloworld'

I have two files in the helloworld directory. app.yaml

application: your-app-id
version: 1
runtime: python27
api_version: 1
threadsafe: true

handlers:
- url: /.*
  script: helloworld.application

and the helloworld.py

import webapp2

class MainPage(webapp2.RequestHandler):

    def get(self):
        self.response.headers['Content-Type'] = 'text/plain'
        self.response.write('Hello, World!')


application = webapp2.WSGIApplication([
    ('/', MainPage),
], debug=True)