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Feature request: Support types annotated with Field for event_handler validation #5953
Description
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
Paramtype (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
- use event_handler OpenAPI Param annotations exclusively
Action = Annotated[FooAction | BarAction, Body(discriminator='action')]- 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
Reacted by Nico Tonnhofer
Activity
leandrodamascena commentedon Jan 31, 2025
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 commentedon Feb 3, 2025
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.
dap0am commentedon Aug 20, 2025
Can I start working on this? I think I have an idea.
leandrodamascena commentedon Aug 20, 2025
Yes @dap0am please go ahead!
feat(event-handler): add support for Pydantic Field discriminator in …
feat(event-handler): add support for Pydantic Field discriminator in …
powertools-for-aws-oss-automation commentedon Sep 11, 2025
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