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feat(event-handler): add support for Pydantic Field discriminator in validation #7227

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@dap0am dap0am commented Aug 21, 2025 •
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Summary

This PR adds support for Pydantic's Field(discriminator='...') syntax in event handler validation, enabling tagged unions to work correctly with the validation middleware.

Fixes #5953

Changes

  • Enhanced get_field_info_annotated_type() in params.py to handle Field(discriminator) + Body() combinations
  • Modified ModelField.__post_init__() in compat.py to preserve discriminator metadata when creating TypeAdapters
  • Added comprehensive tests for discriminator validation and other Field features

Testing

  • Added test_field_discriminator_validation() to verify discriminator functionality with tagged unions
  • Added test_field_other_features_still_work() to ensure other Field features remain functional
  • All existing tests pass without modification

Example Usage

from typing import Literal
from typing_extensions import Annotated
from pydantic import BaseModel, Field
from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.event_handler.openapi.params import Body

app = APIGatewayRestResolver(enable_validation=True)

class FooAction(BaseModel):
    action: Literal["foo"]
    foo_data: str

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

# Now works with Field discriminator
Action = Annotated[FooAction | BarAction, Field(discriminator="action")]

@app.post("/actions")
def create_action(action: Annotated[Action, Body()]):
    return {"received": action.model_dump()}

Checklist

  • Tests are passing
  • Linting and type checking pass
  • Follows project contribution guidelines
  • Maintains backward compatibility

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

@dap0am
feat(event-handler): add support for Pydantic Field discriminator in …
f6ca987 
…validation (aws-powertools#5953)

Enable use of Field(discriminator='...') with tagged unions in event handler validation.
This allows developers to use Pydantic's native discriminator syntax instead of
requiring Powertools-specific Param annotations.

- Handle Field(discriminator) + Body() combination in get_field_info_annotated_type
- Preserve discriminator metadata when creating TypeAdapter in ModelField
- Add comprehensive tests for discriminator validation and Field features
@dap0am dap0am requested a review from a team as a code owner August 21, 2025 08:30
@dap0am dap0am requested a review from anafalcao August 21, 2025 08:30
@pull-request-size pull-request-size bot added the size/L Denotes a PR that changes 100-499 lines, ignoring generated files. label Aug 21, 2025
@leandrodamascena
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Hey @dap0am please run make format to fix the CI.

@dap0am
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dap0am commented Aug 21, 2025

Hey @dap0am please run make format to fix the CI.

Done, thanks.

@leandrodamascena
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Hey @dap0am please run make format to fix the CI.

Done, thanks.

Thanks for fixing! You can run make pr before commit the code. So, you can see if there are errors in the code you refactored.

leandrodamascena
leandrodamascena requested changes Aug 29, 2025
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Hey @dap0am! Please let me know if you need any help here.

@dap0am
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dap0am commented Sep 3, 2025

Hey @dap0am! Please let me know if you need any help here.

Hi @leandrodamascena nope, I am just waiting for your review.

@leandrodamascena
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Hey @dap0am please run make format to fix the CI.

Done, thanks.

Thanks for fixing! You can run make pr before commit the code. So, you can see if there are errors in the code you refactored.

Hi @dap0am, can you fix the CI and make the appropriate changes so as not to break the current tests? The CI is failing because there are some refactorings that can introduce bug regressions into the existing code and break all customers using it.

Use make pr locally to see which tests are failing and what you need to change in your code to avoid breaking the current tests.

leandrodamascena
leandrodamascena requested changes Sep 3, 2025
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@dap0am please fix the CI before we start the review.

@dap0am
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dap0am commented Sep 3, 2025

Hey @dap0am please run make format to fix the CI.

Done, thanks.

Thanks for fixing! You can run make pr before commit the code. So, you can see if there are errors in the code you refactored.

Hi @dap0am, can you fix the CI and make the appropriate changes so as not to break the current tests? The CI is failing because there are some refactorings that can introduce bug regressions into the existing code and break all customers using it.

Use make pr locally to see which tests are failing and what you need to change in your code to avoid breaking the current tests.

Will do that, thanks.

@dap0am
fix(event-handler): preserve FieldInfo subclass types in copy_field_info
ead3ee8 
Fix regression where copy_field_info was losing custom FieldInfo subclass
types (Body, Query, etc.) by using shallow copy instead of from_annotation.
This resolves the failing test_validate_embed_body_param while maintaining
the discriminator functionality.
@dreamorosi
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Hi @dap0am, some of the unit tests are failing - please take a look if you can and also resolve the SonarCloud issues listed in the comment above.

Thanks!

dap0am and others added 2 commits September 4, 2025 15:54
@dap0am
refactor(event-handler): reduce cognitive complexity and address Sona…
e2c8b49 
…rCloud issues

- Refactor get_field_info_annotated_type function by extracting helper functions to reduce cognitive complexity from 29 to below 15
- Fix copy_field_info to preserve FieldInfo subclass types using shallow copy instead of from_annotation
- Rename variable Action to action_type to follow Python naming conventions
- Resolve failing test_validate_embed_body_param by maintaining Body parameter type recognition
- Add helper functions: _has_discriminator, _handle_discriminator_with_body, _create_field_info, _set_field_default
- Maintain full backward compatibility and discriminator functionality
@leandrodamascena
Merge branch 'develop' into develop
d61e531
@sonarqubecloud SonarQubeCloud
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sonarqubecloud bot commented Sep 4, 2025

Quality Gate Passed Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

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@leandrodamascena leandrodamascena leandrodamascena requested changes

@anafalcao anafalcao Awaiting requested review from anafalcao anafalcao is a code owner automatically assigned from aws-powertools/lambda-python-core

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Feature request: Support types annotated with Field for event_handler validation
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@dap0am @leandrodamascena @dreamorosi