AnyOf vs OneOf for Optional[T]

[bruno-f-cruz]

This is a discussion that generates a lot of confusion in the OpenApi and in the pydantic forums (pydantic/pydantic#7161).

Basically, when you have a nullable type, which tag should you use? AnyOf or OneOf?
Pydantic locks it to AnyOf (in a bit of a self-admited arbitrary choice, https://github.com/pydantic/pydantic/blob/da468c48624b202685af4baebf0edf0df4402a81/pydantic/json_schema.py#L1021-L1023) which breaks the interoperability with other tools. This can be bypassed by having our own JsonGenerator:

from pydantic.json_schema import (
    GenerateJsonSchema,
    JsonSchemaValue,
    _deduplicate_schemas,
)
from pydantic_core import core_schema


ref_template = "#/definitions/{model}"

class CustomGenerateJsonSchema(GenerateJsonSchema):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.nullable_as_oneof = kwargs.get("nullable_as_oneof", True)

    def nullable_schema(self, schema: core_schema.NullableSchema) -> JsonSchemaValue:

        null_schema = {"type": "null"}
        inner_json_schema = self.generate_inner(schema["schema"])

        if inner_json_schema == null_schema:
            return null_schema
        else:
            if self.nullable_as_oneof:
                return self.get_flattened_oneof([inner_json_schema, null_schema])
            else:
                return super().get_flattened_anyof([inner_json_schema, null_schema])

    def get_flattened_oneof(self, schemas: list[JsonSchemaValue]) -> JsonSchemaValue:
        members = []
        for schema in schemas:
            if len(schema) == 1 and "oneOf" in schema:
                members.extend(schema["oneOf"])
            else:
                members.append(schema)
        members = _deduplicate_schemas(members)
        if len(members) == 1:
            return members[0]
        return {"oneOf": members}

The export thus becomes:

MyModel.model_json_schema(
                schema_generator=CustomGenerateJsonSchema, ref_template=ref_template
            ),