Using the Local Development Server

The Google Cloud SDK and App Engine SDK for Python each include a local development server that you can run locally to simulate your application running in production App Engine. The simulated environment enforces some sandbox restrictions, such as restricted system functions and Python module imports, but not others, like request time-outs or quotas.

The local development server also simulates the services provided by App Engine Python SDK libraries, including Datastore, Memcache, and Task Queues, by performing their tasks locally. Note that when your application is running in the development server, you can still make remote API calls to the production infrastructure using Google APIs HTTP endpoints.

Running the local development server

Once you have a directory for your application and an app.yaml configuration file, you can start the local development server using the dev_appserver.py command.

dev_appserver.py [PATH_TO_YOUR_APP]

The local server listens on port 8080 by default. You can visit the application at this URL: http://localhost:8080/.

To change which port the local server uses, use the --port option:

dev_appserver.py --port=9999 [PATH_TO_YOUR_APP]

To stop the local server: with Mac OS X or Unix, press Control-C or with Windows, press Control-Break in your command prompt window.

dev_appserver.py not working?

Specifying application IDs

If you want to use application IDs with the local development server, don't use the App ID from the APPLICATION_ID environment variable. The development server differs from production App Engine service in that the local server prepends the string dev~ to the APPLICATION_ID environment variable.

Instead, if you need to access your App ID in the local server, for example to spoof an email address, use the get_application_id() function. To get the hostname of the running app, use the get_default_version_hostname() function.

Detecting application runtime environment

To find out whether your code is running in production or in the local development server, check if os.getenv('SERVER_SOFTWARE', '').startswith('Google App Engine/'). When this is True, you're running in production; otherwise, you're running in the local development server.

Using the local Datastore

The local development server simulates the App Engine datastore using a local file that persists between invocations of the local server.

For more information on indexes and index.yaml, see the Datastore Indexes and Datastore Index Configuration pages.

Browsing the local Datastore

If your app has written data to your local Datastore using the local development server, you can browse it in the local development console.

To browse local Datastore:

1. Start the development server.

2. Access the Datastore Viewer in the local development console. (The URL is http://localhost:8000/datastore.)

3. View your local Datastore contents.

Specifying the ID allocation policy

For production App Engine, you can set the Datastore to automatically generate entity IDs.

Although the auto ID assignment policies for the production server are completely different than those used by the development server, you can also set the automatic ID allocation policy for the local server.

To specify the automatic ID assignment policy, use the --auto_id_policy option:

dev_appserver.py --auto_id_policy=sequential

where --auto_id_policy can be one of the following:

• scattered: (default) IDs are assigned from a non-repeating sequence of approximately uniformly distributed integers.
• sequential: IDs are assigned from the sequence of consecutive integers.

Clearing the local Datastore

To clear the local datastore for an application, invoke the local development server as follows:

dev_appserver.py --clear_datastore=yes [PATH_TO_YOUR_APP]

Changing local Datastore location

To change the location used for the datastore file, use the --datastore_path option:

dev_appserver.py --datastore_path=/tmp/myapp_datastore [PATH_TO_YOUR_APP]

Using the Users service

App Engine provides a Users Service to simplify authentication and authorization for your application. The local development server simulates the behavior of Google Accounts with its own sign-in and sign-out pages. While running under the local development server, the users.create_login_url and users.create_logout_url functions return URLs for /_ah/login and /_ah/logout on the local server.

Using Mail

The local development server can send email for calls to the App Engine mail service using either an SMTP server or a local installation of Sendmail.

dev_appserver.py --smtp_host=smtp.example.com --smtp_port=25 \
  --smtp_user=ajohnson --smtp_password=k1tt3ns [PATH_TO_YOUR_APP]

dev_appserver.py --enable_sendmail=yes

Using URL Fetch

When your application uses the URL fetch API to make an HTTP request, the local development server makes the request directly from your computer. The URL Fetch behavior on the local server may differ from production App Engine if you use a proxy server for accessing websites.

Using the Interactive Console

The Interactive Console allows developers to enter arbitrary Python code into a web form and execute it inside their app's environment; it provides the same access to the application's environment and services as a .py file inside the application itself.

To use the Interactive Console:

1. Start the development server.

2. Access the Interactive console in the in the local development console. (The URL is http://localhost:8000/console.)

3. Enter any Python code you'd like to run in the text area, then submit the form to execute it. For example the following code will add a Datastore entity called Greeting with text content of Hello:

     from google.appengine.ext import ndb
     class Greeting(ndb.Model):
       content = ndb.TextProperty()
   
     e = Greeting(content="Hello")
     e.put()
   

Debugging with PDB

To use the Python PDB debugger:

1. Add this line into your code:

   import pdb; pdb.set_trace();
   

   dev_appserver will break at this point, and drop into the PDB REPL, allowing you to debug your code from the command line.

2. If your application makes multiple simultaneous requests that invoke pdb.set_trace(), multiple debugging sessions will start concurrently, each of which sends output to STDOUT. To avoid this, serialize your requests by disabling the dev_appserver multi-threading and multi-processing support, as follows:

   1. Disable multi-threading for

      • All modules using the --threadsafe_override=false flag.
      • One module using the --threadsafe_override=<MODULENAME>:false flag
      • Multiple modules using the --threadsafe_override=<MODULE1NAME>:false,<MODULE2NAME>:false flag

   2. Disable multi-processing for

      • All modules using the --max_module_instances=1 flag
      • One module using the --max_module_instances=<MODULENAME>:1 flag;
      • Multiple modules using the --max_module_instances=<MODULE1NAME>:1,<MODULE2NAME>:1 flag