Improve application configuration's documentation

[jorgecarleitao]

Is your feature request related to a problem? Please describe.
When a user goes to the documentation on configuration, the first thing that he likely expects is how to configure the application from a deployment's point of view (e.g. SECRET_KEY, DB url, etc), e.g. see starlette's page.

Describe the solution you'd like
It would be interesting to have, as part of the documentation for theconfiguration, something about the deployment configuration (as defined above). If FastAPI does not have any requirement, then it could be as simple as

FastAPI is based on Starlette, and thus you can configure it in the same was as a starlette application, see here for details.

An important part of this aspect, and one that users likely need help with, is how to setup the configuration on a large application with APIRouters. Two examples:

• How to initialize the application depending on the configuration? For users coming from Flask, this would be the create_app part of it.

• How to access configuration values from routes in a module declaring routes for an APIRouter? Should the configuration be a Depend? For users coming from Flask, they are easily accessible through flask.current_app.config, something that does not exist in FastAPI.

[euri10]

the gitter chat had a pretty interesting discussion on that, might be worth checking from May 30 09:54 (sorry cant make a link apparently)

[jekirl]

For settings/config (both in general and with fastapi), I am fan of using BaseSettings from pydantic over starlette's config class

[tiangolo]

Yes, we should have a section in the docs about using Pydantic's BaseSettings.

Let's track the progress on this issue: #190

[Kludex]

This is now documented on https://fastapi.tiangolo.com/advanced/settings/?h=basesettings#create-the-settings-object. 🙏