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# Hello, World!

Python App Engine applications communicate with the web server using the CGI standard. When the server receives a request for your application, it runs the application with the request data in environment variables and on the standard input stream (for POST data). To respond, the application writes the response to the standard output stream, including HTTP headers and content.

Let's begin by implementing a tiny application that displays a short message.

## Creating a Simple Request Handler

Create a directory named `helloworld`. All files for this application reside in this directory.

Inside the `helloworld` directory, create a file named `helloworld.py`, and give it the following contents:

    print 'Content-Type: text/plain'
    print ''
    print 'Hello, world!'

This Python script responds to a request with an HTTP header that describes the content, a blank line, and the message `Hello, world!`.

## Creating the Configuration File

An App Engine application has a configuration file called `app.yaml`. Among other things, this file describes which handler scripts should be used for which URLs.

Inside the `helloworld` directory, create a file named `app.yaml` with the following contents:

    application: helloworld
    version: 1
    runtime: python
    api_version: 1

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

From top to bottom, this configuration file says the following about this application:

- The application identifier is `helloworld`. When you register your application with App Engine in the final step, you will select a unique identifier, and update this value. This value can be anything during development. For now, leave it set to `helloworld`.
- This is version number `1` of this application's code. If you adjust this before uploading new versions of your application software, App Engine will retain previous versions, and let you roll back to a previous version using the administrative console.
- This code runs in the `python` runtime environment, version "1". Additional runtime environments and languages may be supported in the future.
- Every request to a URL whose path matches the regular expression `/.*` (all URLs) should be handled by the `helloworld.py` script.

The syntax of this file is YAML. For a complete list of configuration options, see the app.yaml reference.

## Testing the Application

With a handler script and configuration file mapping every URL to the handler, the application is complete. You can now test it with the web server included with the App Engine SDK.

Start the web server with the following command, giving it the path to the `helloworld` directory:

    google_appengine/dev_appserver.py helloworld/

The web server is now running, listening for requests on port 8080. Test the application by visiting the following URL in your web browser:

- http://localhost:8080/

For more information about running the development web server, including how to change which port it uses, see the Dev Web Server reference, or run the command with the option `--help`.

## Iterative Development

You can leave the web server running while you develop your application. The web server knows to watch for changes in your source files and reload them if necessary.

Try it now: Leave the web server running, then edit `helloworld.py` to change `Hello, world!` to something else. Reload http://localhost:8080/ to see the change.

To shut down the web server, make sure the terminal window is active, then press Control-C (or the appropriate "break" key for your console).

You can leave the web server running for the rest of this tutorial. If you need to stop it, you can restart it again by running the command above.

## Next...

You now have a complete App Engine application! You could deploy this simple greeting right now and share it with users worldwide. But before we deploy it, let's consider using a web application framework to make it easier to add features.

Continue to Using the webapp Framework.