How can I set up environment variables for different stages of my Python application, such as development, testing, and production?

First things first, make sure you create separate configuration files for each development stage, such as `config_dev.py`, `config_test.py`, and `config_prod.py`, where you'll define environment-specific variables and settings. Then, use a tool like `python-decouple` or Python's in-built `os` module to manage the environment variables. Finally, set up an environment variable (ex. `APP_ENV`) to indicate the current stage.

From there, you'll be all set to load the corresponding configuration file based on the value of `APP_ENV` and access environment-specific settings. This approach keeps your sensitive information secure, allowing you to easily switch between configurations for different stages.