How to pass the default value to a variable if None was passed?

You need to enable validate_assignment option in model config:

from typing import Optional

from pydantic import BaseModel, validator


class User(BaseModel):
    name: Optional[str] = ''
    password: Optional[str] = ''

    class Config:
        validate_assignment = True

    @validator('name')
    def set_name(cls, name):
        return name or 'foo'


user = User(name=None, password='some_password', )
print("Name is ", user.name)


user.name = None
print("Name is ", user.name)

Name is  foo
Name is  foo