What does model_validate()'s from_attributes parameter actually do?

[mjog]

Looking at the API docs for model_validate, it says that function "Validate a pydantic model instance", which isn't terribly helpful, but I understand that it roughly "gets model field values from the attributes of the given object".

However what then does from_attributes actually do? By default it is None, so I assume that evaluates to False, but it still seems to get the values from objects regardless. Indeed, setting it true or false doesn't seem to actually change anything, and in any case if the function does not get the model's field values from the attributes of the given object when it is set to false, where else does it get them from?

[Antsthebul]

Came across this while searching and figured I'd answer. This seems to allow models with partially matching fields to be constructed into a new model. Basically what I had was

class ModelA(BaseModel):
    name: str

class ModelB(BaseModel):
    name: str
    age: int

>> model_b = ModelB(name="foo", age=20)
>> model_a = ModelA.model_validate(model_b) # fails w/ Input must be valid dictionary or instance of ModelA
>> model_a = ModelA.model_validate(model_b, from_attributes=False) # <- Good

Same logic is discussed a litter more in detail here https://docs.pydantic.dev/latest/api/config/#pydantic.config.ConfigDict.from_attributes .