Feature request: Support types annotated with Field for event_handler validation

[rtandy]

Use case

Hi, thank you for working on Powertools.

My Pydantic models are shared between multiple applications. For example, a client application builds an instance of a model and submits it to the API, which validates the request against the same model.

A few types I want to accept as API payloads or parameters are annotated with Field, mainly tagged unions.

I am aware of the Powertools OpenAPI Param annotations, but they have downsides:

• having to import Powertools into non-Lambda projects;
• being stuck with a single Param type (i.e. Body, Path, etc) per annotated type, or having to duplicate the arguments for different types

With Powertools 3.5.0, the example code below fails:

• with def create_action(action: Action): AssertionError: Param: action can only be a request body, use Body()
• with def create_action(action: Annotated[Action, Body()]): AssertionError: Only one FieldInfo can be used per parameter

It would be nice if event_handler would allow nested Field annotations, so that I can apply Annotated[some_type, Body()] to an existing annotated type such as a tagged union.

Thank you for considering.

Solution/User Experience

class FooAction(BaseModel):
    action: Literal['foo']
    foo: int 

class BarAction(BaseModel):
    action: Literal['bar']
    bar: int 

Action = Annotated[FooAction | BarAction, Field(discriminator='action')]

app = APIGatewayHttpResolver(enable_validation=True)

@app.post('/actions')
def create_action(action: Action): # or Annotated[Action, Body()]
    ...

def lambda_handler(event, context):
    return app.resolve(event, context)

Alternative solutions

1. use event_handler OpenAPI Param annotations exclusively

Action = Annotated[FooAction | BarAction, Body(discriminator='action')]

2. use TypeAdapter manually

Action = Annotated[FooAction | BarAction, Field(discriminator='action')]
action_adapter = TypeAdapter(Action)

@app.post('/actions')
def create_action():
    try:
        action = action_adapter.validate_json(app.current_event.body)
    except ValidationError as e:
        raise RequestValidationError(e.errors())

Acknowledgment

• This feature request meets Powertools for AWS Lambda (Python) Tenets
• Should this be considered in other Powertools for AWS Lambda languages? i.e. Java, TypeScript, and .NET

[leandrodamascena]

Hi @rtandy! Thanks for opening this issue! Yes, I agree that we need to support this.

Are you interested in submitting a PR to implement this support?

[rtandy]

Are you interested in submitting a PR to implement this support?

I'd like to, but it's not a priority for my employer (the workaround of using a TypeAdapter manually is sufficient) and it will be at least a few weeks before I can look at it on my own time. It's not an urgent request, just an idea for future improvement.