cls is None

cls_is_none.py

from typing import Optional
from pydantic import BaseModel, validator


class MyModel(BaseModel):
    name: Optional[str]

    class Config:
        validate_assignment = True

    @validator('name', always=True)
    def _validate_name(cls, value):
        if value is None:
            return 'hello'
        if cls is None:
            raise ValueError('cls is None')
        return value


if __name__ == '__main__':
    m = MyModel()

    assert m.name == 'hello'
    m.name = 'goodbye'  # <-- error "cls is None"

gistfile1.txt

Traceback (most recent call last):
  File "C:/Users/espdev/.PyCharm2019.3/config/scratches/scratch_17.py", line 24, in <module>
    m.name = 'goodbye'
  File "C:\Users\espdev\AppData\Local\pypoetry\Cache\virtualenvs\lol-ms3eI9mF-py3.8\lib\site-packages\pydantic\main.py", line 289, in __setattr__
    raise ValidationError([error_], type(self))
pydantic.error_wrappers.ValidationError: 1 validation error for MyModel
name
  cls is None (type=value_error)