Assume a REST API which defines a POST method on a resource /foos to create a new Foo. When creating a Foo the name of the Foo is an input parameter (present in the request body). When the server creates a Foo it assigns it an ID. This ID is returned together with the name in the REST response. I am looking for something similar to readOnly in OpenAPI.

The input JSON should look like this:

{"name": "bar"
}

The output JSON should look like that:

{"id": 123,"name": "bar"
}

Is there a way to reuse the same pydantic model? Or is it necessary to use two diffent models?

class FooIn(BaseModel):name: strclass Foo(BaseModel):id: intname: str

I cannot find any mentions of "read only", "read-only", or "readonly" in the pydantic documentation or in the Field class code.

Googling I found a post which mentions

id: int = Schema(..., readonly=True)

But that seems to have no effect in my use case.

It is fine to have multiple models. You can use inheritance to reduce code repetition:

from pydantic import BaseModel# Properties to receive via API create/update
class Foo(BaseModel):name: str# Properties to return via API
class FooDB(Foo):id: int

The documentation which is excellent btw!, goes into this more in-depth.

Here is a real user model example taken from the official full stack project generator. You can see how there are multiple models to define the user schema depending on the context.