How to fix Pydantic "Default value is not JSON serializable" warning when using third-party annotated type? [non-serializable-default]

Pydantic supports annotating third-party types so they can be used directly in Pydantic models and de/serialized to & from JSON.

For example:

from typing import Annotated, Any

from pydantic import BaseModel, model_validator
from pydantic.functional_validators import ModelWrapValidatorHandler
from typing_extensions import Self

# Pretend this is some third-party class
# we can't modify directly...
class Quantity:
    def __init__(self, value: float, unit: str):
        self.value = value
        self.unit = unit

class QuantityAnnotations(BaseModel):
    value: float
    unit: str

    @model_validator(mode="wrap")
    def _validate(value: Any, handler: ModelWrapValidatorHandler[Self]) -> Quantity:
        if isinstance(value, Quantity):
            return value
        validated = handler(value)

        if isinstance(validated, Quantity):
            return validated

        return Quantity(**dict(validated))


QuantityType = Annotated[Quantity, QuantityAnnotations]

class OurModel(BaseModel):
    quantity: QuantityType = Quantity(value=0.0, unit='m')

This works fine, because we just annotated the Quantity type so Pydantic knows how to serialize it to JSON with no issues:

model_instance = OurModel()
print(model_instance.model_dump_json())
# {"quantity":{"value":0.0,"unit":"m"}}

But if we instead try to get the JSON Schema that describes OurModel, we get a warning that it doesn't know how to serialize the default value (the one it just successfully serialized)...

OurModel.model_json_schema()
# ...lib/python3.10/site-packages/pydantic/json_schema.py:2158: PydanticJsonSchemaWarning:
# Default value <__main__.Quantity object at 0x75fcccab1960> is not JSON serializable;
# excluding default from JSON schema [non-serializable-default]

What am I missing here? Is this a Pydantic bug, or do I need to add more to the annotations to tell Pydantic how to serialize the default value in the context of a JSON Schema?

Does anyone have a good workaround to easily include annotated third-party types as default values in JSON Schema generated by Pydantic?

You can provide multiple annotations to Pydantic to help with schema generation, etc.

Below is a full example:

class _QuantityJson(BaseModel):
    value: float
    unit: str

def _validate_quantity(value: Any) -> Quantity:
    if isinstance(value, Quantity):
        return value

    if isinstance(value, dict):
        _qjson = _QuantityJson.model_validate(value)
        return Quantity(value=_qjson.value, unit=_qjson.unit)

    raise ValueError(f"cannot parse quantity from {value}")

_schema_quantity = _QuantityJson.model_json_schema() | {"title": "Quantity"}

def _serialize_quantity(quantity: Quantity):
    return _QuantityJson(value=quantity.value, unit=quantity.unit)

QuantityAnnotated: TypeAlias = Annotated[
    Quantity,
    Field(validate_default=True),
    PlainValidator(_validate_quantity),
    WithJsonSchema(_schema_quantity),
    PlainSerializer(_serialize_quantity),
]

Then you can use it in your model:

class Model(BaseModel):
    quantity: QuantityAnnotated = Field(default=_QuantityJson(value=0.0, unit="m"))

You need to use a _QuantityJson default value because the serializer is not used for the default value (maybe a Pydantic bug?). You can directly assign the default value, but any decent type checker will throw a warning, so I am using a Field with default.

Below is an example usage:

print(Model.model_json_schema())
print(Model.model_validate(dict(quantity=Quantity(3.2, "m"))))
print(Model.model_validate(dict(quantity=Quantity(3.2, "m"))).model_dump_json())
print(Model.model_validate(dict(quantity=dict(value=3.2, unit="m"))))
print(Model.model_validate_json(b"""{"quantity": {"value": 3.3, "unit": "m"}}"""))