Pydantic: cls is None in validator when validate_assignment is True

[espdev]

Bug

When we set config value validate_assignment to True and define custom validator for a field, and try to set value for the field, we get None in cls argument in the validator method instead of expected model class object. In my case I want to store some data in model class object that I want to use for validation and I cannot get it via cls because cls is None. Of course I can use model class object directly, but this behavior is ambiguous and looks like a bug anyway.

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

         pydantic version: 1.3
        pydantic compiled: False
             install path: C:\Users\espdev\AppData\Local\pypoetry\Cache\virtualenvs\ms3eI9mF-py3.8\Lib\site-packages\pydantic
           python version: 3.8.1 (tags/v3.8.1:1b293b6, Dec 18 2019, 23:11:46) [MSC v.1916 64 bit (AMD64)]
                 platform: Windows-10-10.0.18362-SP0
 optional deps. installed: []

A minimal example:

from pydantic import BaseModel, StrictStr, validator

class Model(BaseModel):
    name: StrictStr

    class Config:
        validate_assignment = True

    @validator('name')
    def _validate_foo(cls, value):
        if cls is None:
            raise ValueError('cls is None')
        return value

if __name__ == '__main__':
    m = Model(name='hello')  # OK
    m.name = 'goodbye'       # <-- ValidationError "cls is None"

pydantic.error_wrappers.ValidationError: 1 validation error for Model
name
  cls is None (type=value_error)

[samuelcolvin]

Thanks so much for reporting, amazingly it looks like this has been around for ages and no one noticed, should be fixed in #1174 which I'll release asap.