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# Using the Users Service

Google App Engine provides several useful services based on Google infrastructure, accessible by applications using libraries included with the SDK. One such service is the Users service, which lets your application integrate with Google user accounts. With the Users service, your users can use the Google accounts they already have to sign in to your application.

Let's use the Users service to personalize this application's greeting.

## Using Users

Edit `helloworld/helloworld.py` again, and replace its contents with the following:

    from google.appengine.api import users
    from google.appengine.ext import webapp
    from google.appengine.ext.webapp.util import run_wsgi_app

    class MainPage(webapp.RequestHandler):
      def get(self):
        user = users.get_current_user()

        if user:
          self.response.headers['Content-Type'] = 'text/plain'
          self.response.out.write('Hello, ' + user.nickname())
        else:
          self.redirect(users.create_login_url(self.request.uri))

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

    def main():
      run_wsgi_app(application)

    if __name__ == "__main__":
      main()

Reload the page in your browser. Your application redirects you to the local version of the Google sign-in page suitable for testing your application. You can enter any username you'd like in this screen, and your application will see a fake `User` object based on that username.

When your application is running on App Engine, users will be directed to the Google Accounts sign-in page, then redirected back to your application after successfully signing in or creating an account.

## The Users API

Let's take a closer look at the new pieces:

    user = users.get_current_user()

If the user is already signed in to your application, `get_current_user()` returns the `User` object for the user. Otherwise, it returns `None`.

    if user:
      self.response.headers['Content-Type'] = 'text/plain'
      self.response.out.write('Hello, ' + user.nickname())

If the user has signed in, display a personalized message, using the nickname associated with the user's account.

    else:
      self.redirect(users.create_login_url(self.request.uri))

If the user has not signed in, tell `webapp` to redirect the user's browser to the Google account sign-in screen. The redirect includes the URL to this page (`self.request.uri`) so the Google account sign-in mechanism will send the user back here after the user has signed in or registered for a new account.

For more information about the Users API, see the Users reference.

## Next...

Our application can now greet visiting users by name. Let's add a feature that will let users greet each other.

Continue to Handling Forms With webapp.