Serializing computed fields returning enums

[graydenshand]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

I am unable to serialize a computed field that returns an enum,

Example Code

from enum import Enum
from pydantic import BaseModel, computed_field, field_serializer

class MyEnum(Enum):
    A = "a"
    B = "b"

class MyModel(BaseModel):
    
    @computed_field
    @property
    def computed_a_or_b(self) -> MyEnum:
        return MyEnum.B

    @field_serializer("computed_a_or_b")
    def serialize_my_enum(self, a_or_b: MyEnum) -> str:
        return a_or_b.value


if __name__ == "__main__":
    m = MyModel()
    assert m.model_dump()['computed_a_or_b'] == "b"

The above snippet raises the following error:

KeyError: '__main__.MyEnum:5602143824'

Python, Pydantic & OS Version

pydantic version: 2.7.0
        pydantic-core version: 2.18.1
          pydantic-core build: profile=release pgo=true
                 install path: myproject/.venv/lib/python3.12/site-packages/pydantic
               python version: 3.12.0 (main, Nov 11 2023, 13:36:14) [Clang 15.0.0 (clang-1500.0.40.1)]
                     platform: macOS-14.4.1-arm64-arm-64bit
             related packages: mypy-1.9.0 typing_extensions-4.11.0
                       commit: unknown

[graydenshand]

One other observation to note, if I add a static field to the model with the same enum type, the field_serializer on the computed field works.
e.g. if I add a static field to the above example,

class MyModel(BaseModel):
    static_a_or_b: MyEnum | None = None
    ...

The above snippet succeeds.

[graydenshand]

Another workaround is to annotate the enum with a WrapSerializer.

from enum import Enum
from pydantic import BaseModel, computed_field, WrapSerializer
from typing import Annotated, Any

class MyEnum(Enum):
    A = "a"
    B = "b"

def serialize_my_enum(value: Any, handler, info) ->  str:
    partial_result = handler(value, info)
    return partial_result.value

MySerializableEnum = Annotated[MyEnum, WrapSerializer(serialize_my_enum)]

class MyModel(BaseModel):    
    @computed_field
    @property
    def computed_a_or_b(self) -> MySerializableEnum:
        return MyEnum.B

if __name__ == "__main__":
    m = MyModel()
    assert m.model_dump()['computed_a_or_b'] == "b"

[sydney-runkle]

@graydenshand,

Thanks for reporting. Adding to our 2.7 issues milestone. Looks similar (at least in terms of the error) to #9319.

[sydney-runkle]

Hmm, looks like this issue has been around for a while (not introduced in 2.7). Moving to the 2.8 milestone - still a priority, but not a rush to get out for 2.7.

[NeevCohen]

Hey @sydney-runkle I'd like to work on this if it's still available.