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Middleware

You can add middleware to FastAPI applications.

A "middleware" is a function that works with every request before it is processed by any specific path operation. And also with every response before returning it.

  • It takes each request that comes to your application.
  • It can then do something to that request or run any needed code.
  • Then it passes the request to be processed by the rest of the application (by some path operation).
  • It then takes the response generated by the application (by some path operation).
  • It can do something to that response or run any needed code.
  • Then it returns the response.

!!! note "Technical Details" If you have dependencies with yield, the exit code will run after the middleware.

If there were any background tasks (documented later), they will run *after* all the middleware.

Create a middleware

To create a middleware you use the decorator @app.middleware("http") on top of a function.

The middleware function receives:

  • The request.
  • A function call_next that will receive the request as a parameter.
    • This function will pass the request to the corresponding path operation.
    • Then it returns the response generated by the corresponding path operation.
  • You can then modify further the response before returning it.
{!../../../docs_src/middleware/tutorial001.py!}

!!! tip Keep in mind that custom proprietary headers can be added using the 'X-' prefix.

But if you have custom headers that you want a client in a browser to be able to see, you need to add them to your CORS configurations ([CORS (Cross-Origin Resource Sharing)](cors.md){.internal-link target=_blank}) using the parameter `expose_headers` documented in <a href="https://www.starlette.io/middleware/#corsmiddleware" class="external-link" target="_blank">Starlette's CORS docs</a>.

!!! note "Technical Details" You could also use from starlette.requests import Request.

**FastAPI** provides it as a convenience for you, the developer. But it comes directly from Starlette.

Before and after the response

You can add code to be run with the request, before any path operation receives it.

And also after the response is generated, before returning it.

For example, you could add a custom header X-Process-Time containing the time in seconds that it took to process the request and generate a response:

{!../../../docs_src/middleware/tutorial001.py!}

Other middlewares

You can later read more about other middlewares in the Advanced User Guide: Advanced Middleware{.internal-link target=_blank}.

You will read about how to handle CORS with a middleware in the next section.