An App Engine app is configured using an `app.yaml` file, that contains the runtime, handlers, scaling, and other general settings including environment variables. ## About `app.yaml` files
You can specify the runtime configuration for your Python 3 app, including versions and URLs, in the `app.yaml` file. This file acts as a deployment descriptor of a specific [service](https://docs.cloud.google.com/appengine/docs/standard/an-overview-of-app-engine) version.
You must first create the `app.yaml` file for the `default` service of your app before you can create and deploy `app.yaml` files for additional services. Generally, your `app.yaml` file requires only the `runtime` element for a simple Python 3 app in the standard environment, for example: runtime: python315 App Engine provides default values for all other settings, including the [F1 instance class](https://docs.cloud.google.com/appengine/docs/standard#instance_classes), which determines the memory and CPU resources that are available to your app, and [automatic scaling](https://docs.cloud.google.com/appengine/docs/standard/how-instances-are-managed#scaling_types), which controls how and when new instances of your app are created. If you need to override the default settings, create an `app.yaml` file and specify the settings you need. For more information, see [`app.yaml` file reference](https://docs.cloud.google.com/appengine/docs/standard/reference/app-yaml). You can specify a unique name for your `app.yaml` files, but then you must specify the file name with the [deployment command](https://docs.cloud.google.com/appengine/docs/standard/testing-and-deploying-your-app) as well. For example, if you name your `app.yaml` file `service-name-app.yaml` or `app.standard.yaml`, then you must deploy your app using either: gcloud app deploy service-name-app.yaml gcloud app deploy app.standard.yaml To learn more about structuring multiple services and `app.yaml` files in your app, see [Structuring web services](https://docs.cloud.google.com/appengine/docs/standard/configuration-files).
## All configuration elements For a complete list of all the supported elements in this configuration file, see the [`app.yaml` reference](https://docs.cloud.google.com/appengine/docs/standard/reference/app-yaml).