Error in OpenAPI schema generation with plain serializer when upgrading to pydantic 2.8.0

[rossmacarthur]

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Example Code

from decimal import ROUND_HALF_UP, Decimal
from typing import Annotated, Optional

import fastapi
from pydantic import BaseModel, PlainSerializer


def cents_to_dollars(v: Decimal) -> str:
    v = v * Decimal("0.01")
    v = v.quantize(Decimal("0.00000000"), rounding=ROUND_HALF_UP)
    return format(v, "f")


serializer = PlainSerializer(cents_to_dollars, when_used="json-unless-none")


class Example(BaseModel):
    a: Annotated[Decimal, serializer]  # works fine
    b: Annotated[Optional[Decimal], serializer]  # works fine
    c: Annotated[Optional[Decimal], serializer] = None  # does not work


app = fastapi.FastAPI()


@app.get("/")
def root() -> Example: ...


print(app.openapi())

Description

The above code works fine on pydantic 2.7.4 but errors on pydantic 2.8.0

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.111.0

Pydantic Version

2.8.0

Python Version

3.11.7

Additional Context

No response

[danielqba]

Had the same issue around here, had to pin the pydantic version to 2.7.4

[ceb10n]

Apparently, for some reason that I haven't investigated yet, you need to add SerializationInfo to your serializer:

def cents_to_dollars(v: Decimal, info: SerializationInfo) -> str:
    v = v * Decimal("0.01")
    v = v.quantize(Decimal("0.00000000"), rounding=ROUND_HALF_UP)
    return format(v, "f")

I just saw it in the the Pydantic v2.8 announcement and tryied with your example.

Since it is a problem with your function beeing called with v = None when SerializationInfo is not present, you can validate if v is not None too:

def cents_to_dollars(v: Decimal) -> str:
    if v:
        v = v * Decimal("0.01")
        v = v.quantize(Decimal("0.00000000"), rounding=ROUND_HALF_UP)
        return format(v, "f")

[rossmacarthur]

I think FastAPI is not considering this parameter: when_used="json-unless-none", because when set the serializer is not actually called when the value is None but FastAPI is trying to call it on the default value None in this case.