text: from typing import Optional
  
  from fastapi import FastAPI
  from pydantic import BaseModel
  from starlette.testclient import TestClient
  
  app = FastAPI()
  
  
  class SubModel(BaseModel):
      a: Optional[str] = "foo"
  
  
  class Model(BaseModel):
      x: Optional[int]
      sub: SubModel
  
  
  class ModelSubclass(Model):
      y: int
  
  
  @app.get("/", response_model=Model, response_model_skip_defaults=True)
  def get() -> ModelSubclass:
      return ModelSubclass(sub={}, y=1)
  
  
  client = TestClient(app)
  
  
  def test_return_defaults():
      response = client.get("/")
      assert response.json() == {"sub": {}}
  