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

[fennerm]

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

• [BUG] Null fields dropped from HTTP request bodies #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?