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This appendix describes the installation of the Google App Engine Software Development Kit (SDK) on Linux, plus how to run a simple “Hello, world” application.
This excerpt is from Using Google App Engine. With this book, you can build exciting, scalable web applications quickly and confidently, using Google App Engine -- even if you have little or no experience in programming or web development. Using Google App Engine provides an overview of the tools necessary to use Google App Engine, including Python, HTML, Cascading Style Sheets (CSS), and HTTP. You'll also learn what's required to deploy your applications to Google servers.
The App Engine SDK allows you to run Google App Engine Applications on your local computer. It simulates the runtime environment of the Google App Engine infrastructure.
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
pythonPython 2.5.2 (r252:60911, Feb 22 2008, 07:57:53) ...
The first line will show you the version of Python running on your
exit() to return to the
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”.
In this case, you must download the Linux package onto your computer’s desktop, as shown in 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.4, “Unzipping the file using Terminal” shows how you
can also use the built-in
command-line tool in the Terminal application. Here we are uncompressing
google_appengine_1.1.9.zip (the SDK) to our desktop folder.
Once you have completed the unzipping process, you will get a
google_appengine that contains all the
files necessary to develop your applications, as shown in 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:
Substitute your username instead of
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
ae-01-trivial. The path to this
folder would be
Create a file called
ae-01-trivial folder with
the following contents:
application: ae-01-trivial version: 1 runtime: python api_version: 1 handlers: - url: /.* script: index.py
If you are looking at a PDF copy of this book, please do not copy and paste these lines into your text editor. You might end up with strange characters. Just type them into your editor.
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:
You will be asked if you want App Engine to check for updates
y). After a few messages, the
server will start up, as shown in 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”.
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
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
file, App Engine will not start, and you will see an error as shown in
Figure D.8, “Error in app.yaml”.
In this instance, the mistake is an incorrect indentation of the
final line in
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”.
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”.
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
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.
If you enjoyed this excerpt, buy a copy of Using Google App Engine.
Copyright © 2009 O'Reilly Media, Inc.