mirror of https://github.com/tiangolo/fastapi.git
60 lines
2.9 KiB
Markdown
60 lines
2.9 KiB
Markdown
You can add middleware to **FastAPI** applications.
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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.
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* It takes each **request** that comes to your application.
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* It can then do something to that **request** or run any needed code.
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* Then it passes the **request** to be processed by the rest of the application (by some *path operation*).
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* It then takes the **response** generated by the application (by some *path operation*).
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* It can do something to that **response** or run any needed code.
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* Then it returns the **response**.
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!!! note "Technical Details"
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If you have dependencies with `yield`, the exit code will run *after* the middleware.
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If there were any background tasks (documented later), they will run *after* all the middleware.
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## Create a middleware
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To create a middleware you use the decorator `@app.middleware("http")` on top of a function.
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The middleware function receives:
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* The `request`.
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* A function `call_next` that will receive the `request` as a parameter.
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* This function will pass the `request` to the corresponding *path operation*.
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* Then it returns the `response` generated by the corresponding *path operation*.
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* You can then modify further the `response` before returning it.
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```Python hl_lines="8 9 11 14"
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{!./src/middleware/tutorial001.py!}
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```
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!!! tip
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Have in mind that custom proprietary headers can be added <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers" class="external-link" target="_blank">using the 'X-' prefix</a>.
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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>.
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!!! note "Technical Details"
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You could also use `from starlette.requests import Request`.
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**FastAPI** provides it as a convenience for you, the developer. But it comes directly from Starlette.
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### Before and after the `response`
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You can add code to be run with the `request`, before any *path operation* receives it.
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And also after the `response` is generated, before returning it.
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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:
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```Python hl_lines="10 12 13"
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{!./src/middleware/tutorial001.py!}
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```
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## Other middlewares
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You can later read more about other middlewares in the [Advanced User Guide: Advanced Middleware](../advanced/middleware.md){.internal-link target=_blank}.
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You will read about how to handle <abbr title="Cross-Origin Resource Sharing">CORS</abbr> with a middleware in the next section.
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