To set environment variables for an App Engine application, you need to define them in the app.yaml configuration file. The process is slightly different for the Standard and Flexible environments.

1. App Engine Standard Environment:

For the Standard Environment, define environment variables in the app.yaml file under the env_variables section. Here’s an example:

runtime: python39
instance_class: F1

env_variables:
  API_KEY: 'your_api_key'
  SECRET_KEY: 'your_secret_key'
  ANOTHER_VAR: 'another_value'

handlers:
- url: /.*
  script: auto

Replace your_api_key, your_secret_key, and another_value with the appropriate values for your application.

2. App Engine Flexible Environment:

For the Flexible Environment, define environment variables in the app.yaml file under the env_variables section in the runtime_config. Here’s an example:

runtime: python
env: flex

runtime_config:
  python_version: 3

env_variables:
  API_KEY: 'your_api_key'
  SECRET_KEY: 'your_secret_key'
  ANOTHER_VAR: 'another_value'

entrypoint: gunicorn -b :$PORT main:app

Replace your_api_key, your_secret_key, and another_value with the appropriate values for your application.

After defining the environment variables in the app.yaml file, deploy your App Engine application using the gcloud app deploy command. The environment variables will be available to your application code during runtime.