Installing and Running Google App Engine on Windows XP: Appendix A - Using Google App Engine

Download the appropriate install package for the Google App Engine SDK from http://code.google.com/appengine/downloads.html, as shown in Figure A.1, “Downloading Google Application Engine”.

Figure A.1. Downloading Google Application Engine

Download the Windows installer and double-click on the GoogleApplicationEngine installer; the setup wizard will launch, as shown in Figure A.2, “Installing Google App Engine”.

Figure A.2. Installing Google App Engine

Click through the installation wizard, which should install App Engine. If you do not have Python 2.5, it will install Python 2.5 as well. Once the install is complete, you can discard the downloaded installer.

Let’s create a simple application. Make a folder for your Google App Engine applications. For this example, we’ll use a folder called apps on the desktop: C:\Documents and Settings\csev\Desktop\apps. Make a subfolder in within apps called ae-01-trivial: C:\ Documents and Settings \csev\Desktop\apps\ae-01-trivial.

Using a text editor such as jEdit (http://www.jedit.org), create a file called app.yaml in the ae-01-trivial folder, with the following contents:

application: ae-01-trivial
version: 1
runtime: python
api_version: 1

handlers:
- url: /.*
  script: index.py

Then create a file in the ae-01-trivial folder called index.py, with three lines of Python:

print 'Content-Type: text/plain'
print ''
print 'Hello there Jim'

Then start the Command Prompt program by clicking the Windows icon in the lower left and typing Command in the Start Search box. When it selects the Command Prompt item, press Enter to start the Command Prompt. Use the cd command to navigate into the apps directory, as shown in Figure A.3, “Navigating to the application directory”.

Figure A.3. Navigating to the application directory

Then start the Google App Engine Web Server and run your application using the following command:

\Program Files\Google\google_appengine\dev_appserver.py ae-01-trivial

Once App Engine starts your application, you will be asked if you want App Engine to check for updates (type y) and then, after a few messages, the server will start up, as shown in Figure A.4, “Starting the application server”.

Figure A.4. Starting the application server

The last line tells you which port your application is running on and what URL you should use to access your application. In this case, our application is at http://localhost:8080.

Paste http://localhost:8080 into your browser, and you should see your application, as shown in Figure A.5, “Your Google application”.

Figure A.5. Your Google application

Just for fun, edit index.py to change the name Jim to your own name and refresh the page in the browser to verify your updates.

With two files to edit, there are two general categories of errors that you may encounter. The first common error is making a mistake in your app.yaml file. If you make a mistake in the app.yaml file, App Engine will not start, and you will see an error on the command line, as shown in Figure A.6, “Error in app.yaml”.

Figure A.6. Error in app.yaml

In this instance, the mistake is an incorrect indentation in the last line in the app.yaml (line 8).

When you make a mistake in the app.yaml file, you must the fix the mistake and attempt to start the application again.

The second type of error is in the index.py file. If you make a syntax error in the index.py file, the error will appear in your browser. The error will make it look like everything has gone wrong, as shown in Figure A.7, “Syntax error”.

Figure A.7. Syntax error

Do not be alarmed. Ignore most of the output and scroll to the very bottom of the error output. The error that you need to see is likely to be the very last line of the output. In this case, there was a Python syntax error on the first line of our one-line application, as shown in Figure A.8, “Finding the syntax error”.

Figure A.8. Finding the syntax error

See also http://en.wikipedia.org/wiki/Stack_trace for more information.

If you make a mistake in a file like index.py, you can just fix the file and refresh the page in your browser. There is no need to restart the server.

To shut down the server, go into the window where you started the server and close the window to abort the server. When the server is shut down, you will notice that navigating to http://localhost:8080 will fail because there is no software running on and listening to port 8080.