8000 docs: clarify BaseHTTPMiddleware impact on contextvars for subsequent middleware by yakimka · Pull Request #2943 · encode/starlette · GitHub
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docs: clarify BaseHTTPMiddleware impact on contextvars for subsequent middleware #2943

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docs: clarify BaseHTTPMiddleware impact on contextvars for subsequent middleware #2943

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3ac0e3e
Clarify BaseHTTPMiddleware effect on contextvars propagation
yakimka May 24, 2025
360cf30
Merge branch 'master' into contextvars-middlewre-docs
yakimka Jun 3, 2025
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2 changes: 1 addition & 1 deletion docs/middleware.md
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Expand Up @@ -297,7 +297,7 @@ around explicitly, rather than mutating the middleware instance.

Currently, the `BaseHTTPMiddleware` has some known limitations:

- Using `BaseHTTPMiddleware` will prevent changes to [`contextvars.ContextVar`](https://docs.python.org/3/library/contextvars.html#contextvars.ContextVar)s from propagating upwards. That is, if you set a value for a `ContextVar` in your endpoint and try to read it from a middleware you will find that the value is not the same value you set in your endpoint (see [this test](https://github.com/encode/starlette/blob/621abc747a6604825190b93467918a0ec6456a24/tests/middleware/test_base.py#L192-L223) for an example of this behavior).
- Using `BaseHTTPMiddleware` will prevent changes to [`contextvars.ContextVar`](https://docs.python.org/3/library/contextvars.html#contextvars.ContextVar)s from propagating upwards. That is, if you set a value for a `ContextVar` in your endpoint and try to read it from a middleware you will find that the value is not the same value you set in your endpoint (see [this test](https://github.com/encode/starlette/blob/621abc747a6604825190b93467918a0ec6456a24/tests/middleware/test_base.py#L192-L223) for an example of this behavior). Importantly, this also means that if a `BaseHTTPMiddleware` is positioned earlier in the middleware stack, it will disrupt `contextvars` propagation for any subsequent Pure ASGI Middleware that relies on them.

To overcome these limitations, use [pure ASGI middleware](#pure-asgi-middleware), as shown below.

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