Application sometimes store configuration as constants in the code, which is a violation of strict separation of configuration from code. Where should application configuration be stored?

Environment variables are the standard 12-Factor App method for externalizing configuration because they are language- and OS-agnostic, cannot be accidentally committed to source control, and can be changed per deployment environment without modifying code.

YAML files can still be bundled with the application codebase, making them susceptible to accidental commit and not achieving strict separation from code.

Python libraries are code artifacts and embedding configuration in them directly violates the separation-of-config-from-code principle.

Dockerfiles are part of the build artifact and are committed to source control, making them an inappropriate place for environment-specific configuration.

INI files suffer the same problem as YAML files - they are typically stored alongside the codebase and do not enforce strict separation.