tiangolo.fastapi/docs/tutorial/response-model.md

You can declare the model used for the response with the parameter response_model in any of the path operations:

• @app.get()
• @app.post()
• @app.put()
• @app.delete()
• etc.

{!./tutorial/src/response_model/tutorial001.py!}

!!! note Notice that response_model is a parameter of the "decorator" method (get, post, etc). Not of your path operation function, like all the parameters and body.

It receives a standard Pydantic model and will:

• Convert the output data to the type declarations of the model
• Validate the data
• Add a JSON Schema for the response, in the OpenAPI path operation
• Will be used by the automatic documentation systems

But most importantly:

• Will limit the output data to that of the model. We'll see how that's important below.

Return the same input data

Here we are declaring a UserIn model, it will contain a plaintext password:

{!./tutorial/src/response_model/tutorial002.py!}

And we are using this model to declare our input and the same model to declare our output:

{!./tutorial/src/response_model/tutorial002.py!}

Now, whenever a browser is creating a user with a password, the API will return the same password in the response.

In this case, it might not be a problem, becase the user himself is sending the password.

But if we use sthe same model for another path operation, we could be sending the passwords of our users to every client.

!!! danger Never send the plain password of a user in a response.

Add an output model

We can instead create an input model with the plaintext password and an output model without it:

{!./tutorial/src/response_model/tutorial003.py!}

Here, even though our path operation function is returning the same input user that contains the password:

{!./tutorial/src/response_model/tutorial003.py!}

...we declared the response_model to be our model UserOut, that doesn't include the password:

{!./tutorial/src/response_model/tutorial003.py!}

So, FastAPI will take care of filtering out all the data that is not declared in the output model (using Pydantic).

See it in the docs

When you see the automatic docs, you can check that the input model and output model will both have their own JSON Schema:

And both models will be used for the interactive API documentation:

Recap

Use the path operation decorator's parameter response_model to define response models and especially to ensure private data is filtered out.