[2024-12-20] `pydantic` transforming wrong zero-value container

README.md

Transforming {} to []

$ python transformed.py
{'points': []}
{'points': []}
`grid3` failed as expected
{'points': [{'x': 3, 'y': 4}]}
{'points': [{'x': 3, 'y': 4}, {'x': 5, 'y': 12}]}
`grid6` failed as expected

Allowing polymorphic type but only empty object

$ python polymorphic.py
{'points': []}
{'points': {}}
`grid3` failed as expected
{'points': [{'x': 3, 'y': 4}]}
{'points': [{'x': 3, 'y': 4}, {'x': 5, 'y': 12}]}
`grid6` failed as expected

polymorphic.py

from typing import List, Union

import pydantic


class _EmptyObject(pydantic.BaseModel):
    model_config = pydantic.ConfigDict(extra="forbid")


class Point(pydantic.BaseModel):
    x: int
    y: int


class Grid(pydantic.BaseModel):
    points: Union[List[Point], _EmptyObject]


def main():
    grid1 = Grid.model_validate_json(b'{"points": []}')
    print(grid1.model_dump())
    grid2 = Grid.model_validate_json(b'{"points": {}}')
    print(grid2.model_dump())

    try:
        grid3 = Grid.model_validate_json(b'{"points": {"cannot": "be_here"}}')
        print(grid3.model_dump())
    except pydantic.ValidationError:
        print("`grid3` failed as expected")

    grid4 = Grid.model_validate_json(b'{"points": [{"x": 3, "y": 4}]}')
    print(grid4.model_dump())
    grid5 = Grid.model_validate_json(
        b'{"points": [{"x": 3, "y": 4}, {"x": 5, "y": 12}]}'
    )
    print(grid5.model_dump())

    try:
        grid6 = Grid.model_validate_json(b'{"points": [{"x": 3}]}')
        print(grid6.model_dump())
    except pydantic.ValidationError:
        print("`grid6` failed as expected")


if __name__ == "__main__":
    main()

transformed.py

from typing import Any, List, Union

import pydantic


class Point(pydantic.BaseModel):
    x: int
    y: int


class Grid(pydantic.BaseModel):
    @pydantic.field_validator("points", mode="before")
    @classmethod
    def parse_points(cls, value: Any) -> Any:
        if value == {}:
            return []

        return value

    points: List[Point]


def main():
    grid1 = Grid.model_validate_json(b'{"points": []}')
    print(grid1.model_dump())
    grid2 = Grid.model_validate_json(b'{"points": {}}')
    print(grid2.model_dump())

    try:
        grid3 = Grid.model_validate_json(b'{"points": {"cannot": "be_here"}}')
        print(grid3.model_dump())
    except pydantic.ValidationError:
        print("`grid3` failed as expected")

    grid4 = Grid.model_validate_json(b'{"points": [{"x": 3, "y": 4}]}')
    print(grid4.model_dump())
    grid5 = Grid.model_validate_json(
        b'{"points": [{"x": 3, "y": 4}, {"x": 5, "y": 12}]}'
    )
    print(grid5.model_dump())

    try:
        grid6 = Grid.model_validate_json(b'{"points": [{"x": 3}]}')
        print(grid6.model_dump())
    except pydantic.ValidationError:
        print("`grid6` failed as expected")


if __name__ == "__main__":
    main()