Installing and Running Google App Engine on a Linux System: Appendix D - Using Google App Engine

Google App Engine needs the Python interpreter to work on your system. Python comes usually preinstalled on most Linux distributions. To check whether Python is installed, open a Terminal window and type python to execute the Python interpreter:

python
Python 2.5.2 (r252:60911, Feb 22 2008, 07:57:53)
...

The first line will show you the version of Python running on your system. Type exit() to return to the command line.

If Python is not installed or you have a version other than 2.5.x, you will need to download this version (2.5) of Python from http://python.org. Check to see whether there is a package containing Python 2.5 for your Linux distribution.

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

Figure D.1. Downloading Google Application Engine

In this case, you must download the Linux package onto your computer’s desktop, as shown in Figure D.2, “The App Engine package”.

Figure D.2. The App Engine package

Decompress the downloaded file. In Figure D.3, “Unzipping the file using File Roller”, we are using File Roller to unzip the file.

Figure D.3. Unzipping the file using File Roller

Figure D.4, “Unzipping the file using Terminal” shows how you can also use the built-in unzip command-line tool in the Terminal application. Here we are uncompressing google_appengine_1.1.9.zip (the SDK) to our desktop folder.

Figure D.4. Unzipping the file using Terminal

Once you have completed the unzipping process, you will get a folder named google_appengine that contains all the files necessary to develop your applications, as shown in Figure D.5, “Unzipped content”.

Figure D.5. Unzipped content

The last step is to include the path to Google App Engine scripts into the system’s PATH variable to launch its applications from a terminal using the command:

export PATH=$PATH:/home/computer_name/Desktop/google_appengine/

Substitute your username instead of computer_name.

The export command will work only during your current session. Check your Linux distribution manual to see which file (such as .bashrc_login) you can edit to add this command so that the PATH variable is set properly in each new terminal window that is opened.

We will use this window for the remainder of the steps in this process. If you start a new window, type the previous PATH command once in each new window until you edit your login script to make the path change automatic.

Make a folder for your Google App Engine applications. I am going to put the folder on my desktop and call it apps. The path to this folder is /home/pigui/Desktop/apps. Then make a subfolder in within apps called ae-01-trivial. The path to this folder would be /home/pigui/Desktop/apps/ae-01-trivial.

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 Chuck'

Using the Terminal, navigate to the apps directory, start the Google App Engine Web Server, and run your application using the following command:

dev_appserver.py ae-01-trivial

You will be asked if you want App Engine to check for updates (type y). After a few messages, the server will start up, as shown in Figure D.6, “Starting the application server”.

Figure D.6. 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 D.7, “Your Google application”.

Figure D.7. Your Google application

Just for fun, edit the index.py to change the name “Chuck” to your own name, and refresh the page in your 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 on the app.yaml file, App Engine will not start, and you will see an error as shown in Figure D.8, “Error in app.yaml”.

Figure D.8. Error in app.yaml

In this instance, the mistake is an incorrect indentation of the final line in 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.

If you make a syntax error in the index.py file, the error will appear in your browser. The error looks terrible, like everything went wrong, as shown in Figure D.9, “Syntax error”.

Figure D.9. 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, I made a Python syntax error on the first line of our one-line application, as shown in Figure D.10, “Finding the syntax error”.

Figure D.10. 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 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 press Ctrl-C to abort the server. You should see a message that says something like “Server interrupted by user, terminating,” and the server will shut down. You can start it back up by using the dev_appserver.py command again. 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.