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!!! warning This is a rather advanced topic.
If you are starting with **FastAPI**, you might not need this.
By default, FastAPI will return the responses using Starlette's JSONResponse.
But you can override it.
Use UJSONResponse
For example, if you are squeezing performance, you can use ujson and set the response to be Starlette's UJSONResponse.
Import UJSONResponse
{!./src/custom_response/tutorial001.py!}
!!! note
Notice that you import it directly from starlette.responses, not from fastapi.
Make your path operation use it
Make your path operation use UJSONResponse as the response class using the parameter content_type:
{!./src/custom_response/tutorial001.py!}
!!! info
The parameter is called content_type because it will also be used to define the "media type" of the response.
And will be documented as such in OpenAPI.
HTML Response
To return a response with HTML directly from FastAPI, use HTMLResponse.
Import HTMLResponse
{!./src/custom_response/tutorial002.py!}
!!! note
Notice that you import it directly from starlette.responses, not from fastapi.
Define your content_type class
Pass HTMLResponse as the parameter content_type of your path operation:
{!./src/custom_response/tutorial002.py!}
!!! info
The parameter is called content_type because it will also be used to define the "media type" of the response.
In this case, the HTTP header `Content-Type` will be set to `text/html`.
And it will be documented as such in OpenAPI.
Return a Starlette Response
You can also override the response directly in your path operation.
If you return an object that is an instance of Starlette's Response, it will be used as the response directly.
The same example from above, returning an HTMLResponse, could look like:
{!./src/custom_response/tutorial003.py!}
!!! info
Of course, the Content-Type header will come from the the Response object your returned.
!!! warning
A Response returned directly by your path operation function won't be documented in OpenAPI and won't be visible in the automatic interactive docs.
Document in OpenAPI and override Response
If you want to override the response from inside of the function but at the same time document the "media type" in OpenAPI, you can use the content_type parameter AND return a Response object.
The content_type class will then be used only to document the OpenAPI path operation, but your Response will be used as is.
Return an HTMLResponse directly
For example, it could be something like:
{!./src/custom_response/tutorial004.py!}
In this example, the function generate_html_response() already generates a Starlette Response instead of the HTML in a str.
By returning the result of calling generate_html_response(), you are already returning a Response that will override the default FastAPI behavior.
Declare HTMLResponse as content_type
But by declaring it also in the path operation decorator:
{!./src/custom_response/tutorial004.py!}
OpenAPI knows how to document it
...FastAPI will be able to document it in OpenAPI and in the interactive docs as HTML with text/html:
