Work in progress: Replicate Django-environ FileAwareEnv

[avilaton]

This is a work in progres PR to try to bring FileAwareEnv functionality found in django-environ to this package. Requesting for comments, I still need to polish it a lot, but wanted to get it out there.

The feature allows one to set a variable to indicate that the value should be read from a file instead of from os.environ.

For example, if a variable one passes in is called MAIN, then setting a value for MAIN_FILE to /run/secrets/main will make env.str("MAIN") read the value from that file. Usage is simply switched like this

env = environ.FileAwareEnv()
...

The mentioned functionality uses some caching to avoid reading files every time we use this value, see here
https://github.com/joke2k/django-environ/blob/main/environ/fileaware_mapping.py#L15

Thoughts?

[sloria]

thanks! after reading joke2k/django-environ#328 , i can see the value in this. and i like how you were able to implement this with relatively little code.

just a few things to do before merging:

• merge in the latest dev to fix CI runs
• add docs to README.md. see https://django-environ.readthedocs.io/en/latest/tips.html#docker-style-file-based-variables for inspiration
• flesh out the tests. a few cases that come to mind:
  • value in SECRET_KEY_FILE should take precedence over SECRET_KEY in environment
  • error should be raised if SECRET_KEY_FILE is set to a file that doesn't exist