From 007608519506dc74f6c78254672dfe59cf9bb5ac Mon Sep 17 00:00:00 2001 From: JP-Ellis Date: Wed, 5 Mar 2025 19:04:57 +1100 Subject: [PATCH] docs: add caveat about ContextVar in dependencies As discovered and discussed in #13382, ContextVars set in a _synchronous_ dependency is not available to the rest of the request due to the way FastAPI handles that dependency. The documentation did not make any mention of this, and it was only after I asked that I discovered this was intended behaviour. So I am updating the docs to make this clear to others. Ref: #13382 Signed-off-by: JP-Ellis --- docs/en/docs/tutorial/dependencies/index.md | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/docs/en/docs/tutorial/dependencies/index.md b/docs/en/docs/tutorial/dependencies/index.md index 596ce1599..1bab8ece3 100644 --- a/docs/en/docs/tutorial/dependencies/index.md +++ b/docs/en/docs/tutorial/dependencies/index.md @@ -156,6 +156,12 @@ If you don't know, check the [Async: *"In a hurry?"*](../../async.md#in-a-hurry) /// +/// warning + +If you need to rely on `ContextVar` values across multiple steps in a request, **use an async dependency** or another approach. This is because sync dependencies are executed in the mainthread*, while `async` dependencies run in the same thread pool as the one handling the request. As `ContextVar` provides thread-local storage, any value set inside a sync dependency will not be available in the rest of the request handling. + +/// + ## Integrated with OpenAPI All the request declarations, validations and requirements of your dependencies (and sub-dependencies) will be integrated in the same OpenAPI schema.