When Config.validate_assignment is true, raising an unexpected exception type in a field validator results in the field being removed from model.__dict__

[johnsabath]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Disclaimer

While I understand that the docs state that validators should only raise exceptions of ValueError, TypeError, or AssertionError, it's not unreasonable to assume that other Exception types could be raised by accident/ignorance. Since Pydantic is acting on these unexpected exceptions in a destructive way, I still felt the need to report a bug.

Bug

This is a regression that was introduced with Pydantic 3.7

If a field validator raises an exception that isn't of type ValueError, TypeError, or AssertionError, while Config.validate_assignment is set to True, the field that was being validated is removed from model.__dict__ and can no longer be read or assigned.

Problematic LoC
As far as I can tell, the destructive self.__dict__.pop(name) operation could be replaced with self.__dict__[name].
This way even if an unhandled exception occurs afterwards, the field will still exist on __dict__ and will still have its original value set.

Reproduction of the issue:

from pydantic import validator, BaseModel

class Model(BaseModel):
    foo: str = None

    class Config:
        validate_assignment = True

    @validator('foo')
    def validate_fields(cls, value):
        if value == 'raise_exception':
            raise Exception('Example validator error')
        return value

if __name__ == '__main__':
    model = Model(foo='field1_value')

    print('Model before raising exception in validator')
    print(f'  model={model}')
    print(f'  model.__fields__={model.__fields__}')
    print(f'  model.__dict__={model.__dict__}')

    try:
        model.foo = 'raise_exception'
    except Exception:
        print('Caught exception as expected')

    print('Model after raising exception in validator')
    print(f'  model={model}')
    print(f'  model.__fields__={model.__fields__}')
    print(f'  model.__dict__={model.__dict__}')

    # Raises KeyError: 'foo'
    # File "pydantic/main.py", line 387, in pydantic.main.BaseModel.__setattr__
    model.foo = 'field1_value'

Output:

Model before raising exception in validator
  model=foo='field1_value'
  model.__fields__={'foo': ModelField(name='foo', type=Optional[str], required=False, default=None)}
  model.__dict__={'foo': 'field1_value'}
Caught exception as expected
Model after raising exception in validator
  model=
  model.__fields__={'foo': ModelField(name='foo', type=Optional[str], required=False, default=None)}
  model.__dict__={}
Traceback (most recent call last):
  File "/Users/john.sabath/random/pydantic-issue/main.py", line 45, in <module>
    model.foo = 'field1_value'
  File "/Users/john.sabath/venv/pydantic-issue/lib/python3.7/site-packages/pydantic/main.py", line 387, in __setattr__
    original_value = self.__dict__.pop(name)
KeyError: 'foo'