Best practices to distribute reusable BaseSettings extra sources

[nymous]

Hi!

I originally posted this as a comment in #2106 (comment), but it probably deserves its own question.

I read the issue #2106 and the PR #2107 that got merged in Pydantic 1.8 with great interest, as I had exactly the same problem (thank you so much for this work 🎉).

I wanted to ask the same question almost a year ago, when I started working on a POC to extend Pydantic settings with Hashicorp Vault (see https://github.com/nymous/pydantic-vault). I ended up overloading the _build_values method, thinking it would be relatively safe as this was the only method I changed, but it broke with Pydantic 1.7 and the support for Docker secrets.
I have found this other project that went the same route https://github.com/kewtree1408/pydantic-azure-secrets, I should point them to here and tell them about the new way ^^

I still have a few questions: now that we can extend how Pydantic loads variables, what would be the best way to write a reusable provider (to be published on PyPI)? Should I just export a vault_config_settings_source() function that users can insert in their own BaseSettings class? Should I also provide a VaultBaseSettings class that they can inherit from, and that already has this custom source configured? For the properties, should I keep using the kwargs from the Field() function as I have written in my readme, or subclass it as VaultField?
Finally, how safe will this source customization be regarding to Pydantic updates? From the example in the PR https://github.com/samuelcolvin/pydantic/pull/2107/files#diff-401519a4e7dfa8678034b93ab8841dc323994f8e4457151118a02814d53aaaaa I can see that the customise_sources method takes init_settings, env_settings and file_secret_settings arguments, what will happen if Pydantic ever supports another way of loading values?

Thank you for reading, and thanks to everyone who makes Pydantic awesome ❤️