[BUG] Patch operations reset fields to default if not specified

When using the Python generator, PATCH requests can reset fields to their default values even when the client did not explicitly set those fields. This happens specifically if a default value is specified in the PatchedModel spec. Even though we are defining this default, I don't think it matches user expectations for the value to be implicitly set in the patch request. Openapigenerator 5.4.0 did not exhibit this behavior

openapi-generator version

7.22.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Example API
  version: 1.0.0
paths:
  /users/{id}:
    patch:
      operationId: users_partial_update
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: integer
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/PatchedUser'
      responses:
        '200':
          description: Success
components:
  schemas:
    PatchedUser:
      type: object
      properties:
        name:
          type: string
          default: ""
        status:
          type: string
          default: "active"
        priority:
          type: integer
          default: 0

Generated SDK model:

class PatchedUser(BaseModel):
    """
    PatchedUser
    """ # noqa: E501
    name: Optional[StrictStr] = ''
    status: Optional[StrictStr] = 'active'
    priority: Optional[StrictInt] = 0
    __properties: ClassVar[List[str]] = ["name", "status", "priority"]
    model_config = ConfigDict(
        populate_by_name=True,
        validate_assignment=True,
        protected_namespaces=(),
    )
    def to_dict(self) -> Dict[str, Any]:
        """Return the dictionary representation of the model using alias."""
        excluded_fields: Set[str] = set([
        ])
        _dict = self.model_dump(
            by_alias=True,
            exclude=excluded_fields,
            exclude_none=True,  # <-- The problem: doesn't exclude unset fields with defaults
        )
        return _dict

Steps to reproduce

openapi-generator generate -i openapi-bug-report.yaml -g python -o ./bug-report-sdk --package-name bug_report_sdk

Related issues/PRs

• #19067

Suggest a fix/enhancement

Maybe it would make sense to use exclude_unset=True instead of exclude_none=True when serializing Pydantic models for requests?

Start with the Python generator entry point and the generated model's to_dict method shown in the report, then reproduce the issue with the supplied OpenAPI declaration and generation command. Confirm that a PATCH model with defaulted but unset fields omits those fields from the request payload while explicitly provided values remain serialized.