Return a Response Directly¶

When you create a FastAPI path operation you can normally return any data from it: a dict, a list, a Pydantic model, a database model, etc.

If you declare a Response Model FastAPI will use it to serialize the data to JSON, using Pydantic.

If you don’t declare a response model, FastAPI will use the jsonable_encoder explained in JSON Compatible Encoder and put it in a JSONResponse.

You could also create a JSONResponse directly and return it.

/// tip

You will normally have much better performance using a Response Model than returning a JSONResponse directly, as that way it serializes the data using Pydantic, in Rust.

///

Return a Response¶

You can return a Response or any sub-class of it.

/// info

JSONResponse itself is a sub-class of Response.

///

And when you return a Response, FastAPI will pass it directly.

It won’t do any data conversion with Pydantic models, it won’t convert the contents to any type, etc.

This gives you a lot of flexibility. You can return any data type, override any data declaration or validation, etc.

It also gives you a lot of responsibility. You have to make sure that the data you return is correct, in the correct format, that it can be serialized, etc.

Using the jsonable_encoder in a Response¶

Because FastAPI doesn’t make any changes to a Response you return, you have to make sure its contents are ready for it.

For example, you cannot put a Pydantic model in a JSONResponse without first converting it to a dict with all the data types (like datetime, UUID, etc) converted to JSON-compatible types.

For those cases, you can use the jsonable_encoder to convert your data before passing it to a response:

/// note | Technical Details

You could also use from starlette.responses import JSONResponse.

FastAPI provides the same starlette.responses as fastapi.responses just as a convenience for you, the developer. But most of the available responses come directly from Starlette.

///

Returning a custom Response¶

The example above shows all the parts you need, but it’s not very useful yet, as you could have just returned the item directly, and FastAPI would put it in a JSONResponse for you, converting it to a dict, etc. All that by default.

Now, let’s see how you could use that to return a custom response.

Let’s say that you want to return an XML response.

You could put your XML content in a string, put that in a Response, and return it:

How a Response Model Works¶

When you declare a Response Model - Return Type in a path operation, FastAPI will use it to serialize the data to JSON, using Pydantic.

As that will happen on the Rust side, the performance will be much better than if it was done with regular Python and the JSONResponse class.

When using a response_model or return type, FastAPI won’t use the jsonable_encoder to convert the data (which would be slower) nor the JSONResponse class.

Instead it takes the JSON bytes generated with Pydantic using the response model (or return type) and returns a Response with the right media type for JSON directly (application/json).

Notes¶

When you return a Response directly its data is not validated, converted (serialized), or documented automatically.

But you can still document it as described in Additional Responses in OpenAPI.

You can see in later sections how to use/declare these custom Responses while still having automatic data conversion, documentation, etc.