pydantic + fastapi response schema failed to be validated

Question

schemas.py

class ExpenseBase(BaseModel):
    Fixed: float
    Variable: float
    Salary: float
    month: int
    year: int

class Expense(ExpenseBase):
    class Config:
        arbitrary_types_allowed = True
        orm_mode = True

class ExpenseSingle(BaseModel):
    __root__: Expense
    class Config:
        arbitrary_types_allowed = True
        orm_mode = True

In my main.py :

from fastapi import Depends, FastAPI, HTTPException
from fastapi.middleware.cors import CORSMiddleware

from sqlalchemy.orm import Session

from . import api, models, schemas
from .database import SessionLocal, engine

[...]

@app.get('/expense/{id}/', response_model=schemas.ExpenseSingle)
def read_expense(id: int, db: Session = Depends(get_db)):
   
    if id is None or isinstance(id, int) is not int:
        raise HTTPException(status_code=400, detail="ID is not int")

    expense = api.get_expense_by_id(db, id=id)
    return expense

but I get the following error when I try to get that API endpoint:

pydantic.error_wrappers.ValidationError: 1 validation error for ExpenseSingle
response
  none is not an allowed value (type=type_error.none.not_allowed)

I have tried to change schemas.py , to this:

class ExpenseSingle(BaseModel):
    reponse: None
    class Config:
        arbitrary_types_allowed = True
        orm_mode = True

I can't understand where it should be fixed - looks like I didn't understand how pydantic and fastapi relate to each other...

Answer 1

This was solved when I started to handle empty ( None ) responses from the database with the following code:

if not expense:
        raise HTTPException(status_code=404, detail="No Expenses found")

return expense