Pydantic validators are great! The validation is performed as soon as you instantiate your model.

But how do we perform validation on modification of a particular attribute or field of the (model) instance after its creation? When we instantiate our model, all the validators run. But when we change the value of a particular (model) instance attribute or field, no associated validation is performed by default.

The answer to this problem is the validate_assignment config option.

from pydantic import BaseModel, ValidationError, validator

class UserModel(BaseModel):
    name: str
    username: str

    @validator('name')
    def name_must_contain_space(cls, v, values):
        if ' ' not in v:
            raise ValueError('must contain a space')
        
        return v.title()
    
    class Config:
        # This is what you need!
        validate_assignment = True

try:
    user = UserModel(
        name='samuel colvin',
        username='scolvin'
    )
    user.name = 'samualcolvin' # Performs validation due to validate_assignment=True
    print(user)
except ValidationError as e:
    print(e)
    """
    1 validation error for UserModel
    name
        must contain a space (type=value_error)
    """

As you can see in the example above, when the user.name attribute is changed, it triggers all the associated validators with it and raises an exception.