[SOLVED] VSCode Pylance Errors on FastAPI Response Type that Converts

VSCode Pylance Errors on FastAPI Response Type that Converts

I'm using VSCode and the default Python Language Support extension from MS, whcih lints with Pylance. I have a Python 3.12 FastAPI project which uses type annotations to indicate which models should be returned as responses from the handlers using the pattern suggested by the FastAPI docs (https://fastapi.tiangolo.com/tutorial/response-model/#return-type-and-data-filtering). There are no errors when the code is run and the app works correctly, returning the correct results in the correct models. However, I get Pylance linting errors that my function return types are not compatible with my returns.

Here are my FastAPI SQLModel/Pydantic models:

class PetBase(SQLModel):
    name: str

class Pet(PetBase, table=True):
    id: str

class PetOutgoing(PetBase):
    id: str

Here's my handler:

@router.get("/{pet_id}")
def retrieve_one(pet_id: str) -> PetOutgoing:
    pet: Pet = get_pet_by_id(pet_id)
    return pet

This endpoint returns the PetOutgoing model. FastAPI converts it on the fly. But here's the Pylance linting error I get.

Expression of type "Pet" is incompatible with return type "PetOutgoing"
  "Pet" is incompatible with "PetOutgoing" PylancereportReturnType
(variable) pet: Pet

What am I doing wrong?

Solution

The answer is to type hint the handler function with the type the function returns and add the outgoing model you want to the handler decorator, using the response_model parameter.

@router.get("/{pet_id}", response_model=PetOutgoing)
def retrieve_one(pet_id: str) -> Pet:
    pet: Pet = get_pet_by_id(pet_id)
    return pet

This lints correctly and returns the correct model.
See docs: https://fastapi.tiangolo.com/tutorial/response-model/