Bug: `@computed_field` properties are missing in generated OpenAPI schemas

[jontmy]

Expected Behaviour

Define a Pydantic model with a @computed_field and use it as the response object, etc.

class EntityWithComputedField(BaseModel):
    foo: int = 0

    @computed_field
    @property
    def foo_plus_one(self) -> int:
        return self.foo + 1

The generated OpenAPI schema should include the field foo_plus_one, for example in this snippet of an OpenAPI schema:

"components": {
    "schemas": {
      "EntityWithComputedField": {
        "properties": {
          "foo": {
            "type": "integer",
            "title": "Foo",
            "default": 0
          },
          "foo_plus_one": {
            "type": "integer",
            "title": "Foo Plus One",
            "readOnly": true
          }
        },
        "type": "object",
        "required": [
          "foo_plus_one"
        ],
        "title": "EntityWithComputedField"
      },

Similar discussion in FastAPI (now supported): fastapi/fastapi#9856.

Current Behaviour

The generated OpenAPI schema does not include the field foo_plus_one.

Snippet of the generated schema:

"components": {
    "schemas": {
      "EntityWithComputedField": {
        "properties": {
          "foo": {
            "type": "integer",
            "title": "Foo",
            "default": 0
          }
        },
        "type": "object",
        "title": "EntityWithComputedField"
      },

Code snippet

from aws_lambda_powertools.event_handler import HttpResolverLocal
from aws_lambda_powertools.utilities.typing import LambdaContext
from pydantic import BaseModel, computed_field

app = HttpResolverLocal(enable_validation=True, debug=False)
app.enable_swagger(path="/swagger")


class EntityWithComputedField(BaseModel):
    foo: int = 0

    @computed_field
    @property
    def foo_plus_one(self) -> int:
        return self.foo + 1


def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)


@app.get("/")
def hello() -> EntityWithComputedField:
    return EntityWithComputedField().model_dump()

Possible Solution

Powertools should set the JSON schema mode during schema generation to "serialization" instead of "validation" (which is the default).

Reference: https://pydantic.dev/docs/validation/latest/concepts/json_schema/

Steps to Reproduce

In a terminal, run:

mkdir openapi-computed-field
cd openapi-computed-field
uv init --build-backend uv
uv add aws-lambda-powertools pydantic uvicorn
mkdir -p src/openapi_computed_field
touch src/openapi_computed_field/main.py

Update the created src/openapi_computed_field/main.py file with the following:

from aws_lambda_powertools.event_handler import HttpResolverLocal
from aws_lambda_powertools.utilities.typing import LambdaContext
from pydantic import BaseModel, computed_field

app = HttpResolverLocal(enable_validation=True, debug=False)
app.enable_swagger(path="/swagger")


class EntityWithComputedField(BaseModel):
    foo: int = 0

    @computed_field
    @property
    def foo_plus_one(self) -> int:
        return self.foo + 1


def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)


@app.get("/")
def hello() -> EntityWithComputedField:
    return EntityWithComputedField().model_dump()

Back in the terminal, run:

uv run uvicorn openapi_computed_field.main:app

Navigate to http://127.0.0.1:8000/swagger and notice how the EntityWithComputedField schema does not include the computed field foo_plus_one.

Powertools for AWS Lambda (Python) version

latest

AWS Lambda function runtime

3.14

Packaging format used

PyPi

Debugging logs

[leandrodamascena]

Hey @jontmy thanks for opening this issue. I'll take a look.