The best practice for separating application configuration from code, as advocated by the Twelve-Factor App methodology, is to store it in environment variables. This approach ensures a clean decoupling of the application from its deployment environment. Configuration stored in the environment can be easily changed between deployments without any code modifications. This method enhances portability, as the same application artifact (e.g., a container image) can be promoted through different environments (development, staging, production) by simply setting the appropriate environment variables at runtime. It also improves security by preventing sensitive credentials from being committed to a code repository.

B. YAML files: While common, config files can be accidentally committed to version control and often lead to environment-specific files, which is a less clean separation than environment variables.

C. Python libraries: Storing configuration in a library is a form of hardcoding, directly violating the principle of separating configuration from the application code.

D. Dockerfiles: Baking configuration into a Dockerfile creates an inflexible container image that is tied to a specific environment, preventing the reuse of the same image across different stages.

E. INI files: Similar to YAML files, INI files are a file-based configuration method that is considered inferior to environment variables for modern, cloud-native applications.

1. The Twelve-Factor App

III. Config: "The twelve-factor app stores config in environment variables... Unlike config files

they are a language- and OS-agnostic standard. Another aspect of config management is grouping. Sometimes apps group config into named groups... A twelve-factor app is opposed to this grouping of configs

and instead encourages tools to manage these values separately for each deploy."

Source: Heroku

"The Twelve-Factor App

" Section III. Config. Retrieved from https://12factor.net/config

2. Kubernetes Documentation

Configure a Pod to Use a ConfigMap: "A common usage pattern is to set environment variables for a Pod using data from a ConfigMap... This makes your application configuration portable." This official documentation for a core container orchestration platform demonstrates the implementation of the environment variable pattern.

Source: Kubernetes Authors

"Configure a Pod to Use a ConfigMap

" Kubernetes Documentation

spec.containers[].envFrom and spec.containers[].env sections.

3. Docker Documentation

Docker run reference: "With the -e

--env

and --env-file flags

you can set environment variables for the process running inside the container... This allows you to run the same image in development

test

and production with different configurations."

Source: Docker Inc.

"Docker run reference

" Docker Docs

Environment variables (-e

--env

--env-file) section.