Pydantic validation errors with None values

I have a returned result from a webservice, whose values come as they are (see example). The keys are not Optional and must be included.

2 validation errors for Result:

• cause – str type expected (type=type.error.str)
• urls – str type expected (type=type.error.str)

{ 'code': 0, 'codetext': 'blablabla', 'status': None, 'cause': None, 'urls': None }  

class Result(BaseModel):
  
  code: int
  codetext: str
  status: str = ""
  cause: str = ""
  urls: str = ""

  @validator("status", "cause", "urls")
  def replace_none(cls, v):
    return v or "None"

First questions is, what is the correct way to handle the None values coming as answer from the webservice and set to ‘None’ (with single-quotation marks and None as String) and secondly, why are there only 2 and not 3 errors?

Answer

You can mark fields as required but optional by declaring the field optional and using ellipsis (...) as its value. Here is a complete, simplified example with tests to verify it works (execute with pytest <file>):

import pytest
from pydantic import BaseModel, ValidationError, validator


class Result(BaseModel):
    code: int
    status: str | None = ...

    @validator("status")
    def replace_none(cls, v):
        return v or "None"


def test_result():
    p = Result.parse_obj({"code": 1, "status": "test status"})
    assert p.code == 1
    assert p.status == "test status"


def test_required_optionals():
    """Test that missing fields raise an error."""
    with pytest.raises(ValidationError):
        Result.parse_obj({"code": 1})


def test_required_optional_accepts_none():
    """Test that none values are accepted."""
    p = Result.parse_obj(
        {
            "code": 0,
            "status": None,
        }
    )
    assert p.status == "None"