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      1 .. _tutorials.gettingstarted.usingusers:
      2 
      3 Using the Users Service
      4 =======================
      5 Google App Engine provides several useful services based on Google
      6 infrastructure, accessible by applications using libraries included with the
      7 SDK. One such service is the Users service, which lets your application
      8 integrate with Google user accounts. With the Users service, your users can
      9 use the Google accounts they already have to sign in to your application.
     10 Let's use the Users service to personalize this application's greeting.
     11 
     12 
     13 Using Users
     14 -----------
     15 Edit ``helloworld/helloworld.py`` again, and replace its contents with the
     16 following::
     17 
     18     from google.appengine.api import users
     19     import webapp2
     20 
     21     class MainPage(webapp2.RequestHandler):
     22         def get(self):
     23             user = users.get_current_user()
     24 
     25             if user:
     26                 self.response.headers['Content-Type'] = 'text/plain'
     27                 self.response.out.write('Hello, ' + user.nickname())
     28             else:
     29                 self.redirect(users.create_login_url(self.request.uri))
     30 
     31     application = webapp2.WSGIApplication([
     32         ('/', MainPage)
     33     ], debug=True)
     34 
     35     def main():
     36         application.run()
     37 
     38     if __name__ == "__main__":
     39         main()
     40 
     41 Reload the page in your browser. Your application redirects you to the local
     42 version of the Google sign-in page suitable for testing your application.
     43 You can enter any username you'd like in this screen, and your application
     44 will see a fake ``User`` object based on that username.
     45 
     46 When your application is running on App Engine, users will be directed to the
     47 Google Accounts sign-in page, then redirected back to your application after
     48 successfully signing in or creating an account.
     49 
     50 
     51 The Users API
     52 -------------
     53 Let's take a closer look at the new pieces.
     54 
     55 If the user is already signed in to your application, ``get_current_user()``
     56 returns the ``User`` object for the user. Otherwise, it returns ``None``::
     57 
     58     user = users.get_current_user()
     59 
     60 If the user has signed in, display a personalized message, using the nickname
     61 associated with the user's account::
     62 
     63     if user:
     64         self.response.headers['Content-Type'] = 'text/plain'
     65         self.response.out.write('Hello, ' + user.nickname())
     66 
     67 If the user has not signed in, tell ``webapp2`` to redirect the user's browser
     68 to the Google account sign-in screen. The redirect includes the URL to this
     69 page (``self.request.uri``) so the Google account sign-in mechanism will send
     70 the user back here after the user has signed in or registered for a new
     71 account::
     72 
     73     self.redirect(users.create_login_url(self.request.uri))
     74 
     75 For more information about the Users API, see the `Users reference <http://code.google.com/appengine/docs/python/users/>`_.
     76 
     77 
     78 Next...
     79 -------
     80 Our application can now greet visiting users by name. Let's add a feature that
     81 will let users greet each other.
     82 
     83 Continue to :ref:`tutorials.gettingstarted.handlingforms`.
     84