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.
helloworld directory, create a file named
helloworld.py, and give it the following contents:
Run / Modify"""A simple webapp2 server.""" import webapp2 class MainPage(webapp2.RequestHandler): def get(self): self.response.headers['Content-Type'] = 'text/plain' self.response.write('Hello, World!') application = webapp2.WSGIApplication([ ('/', MainPage), ], debug=True)
This Python script responds to a request with an HTTP header that describes the content and the message
Note: Ensure that you save the files you create as plain text. You may encounter errors otherwise.
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.
helloworld directory, create a file named
app.yaml with the following contents:
Run / Modifyapplication: your-app-id version: 1 runtime: python27 api_version: 1 threadsafe: true handlers: - url: /.* script: helloworld.application
From top to bottom, this configuration file says the following about this application:
- The application identifier is
your-app-id. Every new application on App Engine has a unique application identifier. You'll choose the identifier for your application when you register it in the next step. Until then you can just leave the value here set to
your-app-idbecause this value is not important when developing locally.
- This is version number
1of 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
python27runtime environment, API version
1. Additional runtime environments and languages may be supported in the future.
- This application is
threadsafeso the same instance can handle several simultaneous requests. Threadsafe is an advanced feature and may result in erratic behavior if your application is not specifically designed to be threadsafe.
- Every request to a URL whose path matches the regular expression
/.*(all URLs) should be handled by the
applicationobject in the
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.
If you're using the Google App Engine Launcher, you can set up the application by selecting the File menu, Add Existing Application..., then selecting the
helloworld directory. Select the application in the app list, click the Run button to start the application, then click the Browse button to view it. Clicking Browse simply loads (or reloads) http://localhost:8080/ in your default web browser.
If you're not using Google App Engine Launcher, start the web server with the following command, giving it the path to the
The web server is now running, listening for requests on port 8080. You can test the application by visiting the following URL in your web browser:
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
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/ or click Browse in Google App Engine Launcher 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), or click Stop in Google App Engine Launcher.
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.
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 take a closer look at the webapp2 framework and add some more interesting features.
Continue to Explaining the webapp2 Framework.