Python 3.9 is now generally available.

Using the local development server

The Cloud SDK includes a local development server (dev_appserver.py). You can use the local development server to simulate running your application in production App Engine as well as use it to access App Engine bundled services.

The simulated environment enforces some sandbox restrictions, such as restricted system functions and Python 3 module imports, but not others, like request time-outs or quotas.

The local development server also simulates the services provided by the libraries in the SDK for App Engine, including Datastore, Memcache, and Task Queues, by performing their tasks locally. 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

After you create the app.yaml configuration file for your app, you can start the local development server with the dev_appserver.py command to run your app locally.

To start the local development server:

  1. Run the dev_appserver.py command as follows from the directory that contains your app's app.yaml configuration file:

    Specify the directory path to your app, for example:

    dev_appserver.py --runtime_python_path=/usr/bin/python3 [PATH_TO_YOUR_APP]

    You can also set the argument to a comma-separated key-value list like this:

    dev_appserver.py --runtime_python_path="python27=/user/bin/python2.7,python3=/usr/bin/python3" [PATH_TO_YOUR_APP]

    Alternatively, you can specify the configuration file of a specific service, for example:

    dev_appserver.py --runtime_python_path=/usr/bin/python3 app.yaml

    To change the port, you include the --port option:

    dev_appserver.py  --runtime_python_path=/usr/bin/python3 --port=9999 [PATH_TO_YOUR_APP]

    Is dev_appserver.py not working?

    To learn more about the dev_appserver.py command options, see Local Development Server Options.

  2. The local development server is now running and listening for requests. You can visit http://localhost:8080/ in your web browser to see the app in action.

    If you specified a custom port with the --port option, remember to open your browser to that port.

To stop the local server from the command line, you press the following:

  • macOS or Linux: Control-C
  • Windows: Control-Break

Specifying application IDs

To access your App ID in the local server, for example to spoof an email address, use the function. To get the hostname of the running app, use the function.

Detecting application runtime environment

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 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.

Using SMTP

To enable mail support with an SMTP server, invoke dev_appserver.py as follows::

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

where you set the --smtp_host, --smtp_port, --smtp_user and --smtp_password options with your own values.

Using Sendmail

To enable mail support with Sendmail, invoke dev_appserver.py as follows:

dev_appserver.py --enable_sendmail=yes [PATH_TO_YOUR_APP]

The local server will use the sendmail command to send email messages with your installation's default configuration.

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.