Pydantic validations for extra fields that not defined in schema

Question

I am using pydantic for schema validations and I would like to throw an error when any extra field that isn't defined is added to a schema.

from typing import Literal, Union

from pydantic import BaseModel, Field, ValidationError


class Cat(BaseModel):
    pet_type: Literal['cat']
    meows: int


class Dog(BaseModel):
    pet_type: Literal['dog']
    barks: float


class Lizard(BaseModel):
    pet_type: Literal['reptile', 'lizard']
    scales: bool


class Model(BaseModel):
    pet: Union[Cat, Dog, Lizard] = Field(..., discriminator='pet_type')
    n: int


print(Model(pet={'pet_type': 'dog', 'barks': 3.14, 'eats': 'biscuit'}, n=1))
""" try:
    Model(pet={'pet_type': 'dog'}, n=1)
except ValidationError as e:
    print(e) """

In the above code, I have added the eats field which is not defined. The pydantic validations are applied and the extra values that I defined are removed in response. I want to throw an error saying eats is not allowed for Dog or something like that. Is there any way to achieve that?

And is there any chance that we can provide the input directly instead of the pet object?
print(Model({'pet_type': 'dog', 'barks': 3.14, 'eats': 'biscuit', n=1})). I tried without descriminator but those specific validations are missing related to pet_type. Can someone guide me how to achieve either one of that?

Answer 1

Pydantic v2

You can use the extra field in the model_config class attribute to forbid extra attributes during model initialisation (by default, additional attributes will be ignored).

For example:

from pydantic import BaseModel, ConfigDict

class Pet(BaseModel):
    model_config = ConfigDict(extra="forbid")

    name: str

data = {
    "name": "some name",
    "some_extra_field": "some value",
}

my_pet = Pet.model_validate(data)   # <- effectively the same as Pet(**pet_data)

will raise a ValidationError:

ValidationError: 1 validation error for Pet
some_extra_field
  Extra inputs are not permitted [type=extra_forbidden, input_value='some value', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/extra_forbidden

Works as well when the model is "nested", e.g.:

class PetModel(BaseModel):
    my_pet: Pet
    n: int

pet_data = {
    "my_pet": {"name": "Some Name", "invalid_field": "some value"},
    "n": 5,
}

pet_model = PetModel.model_validate(pet_data)
# Effectively the same as
# pet_model = PetModel(my_pet={"name": "Some Name", "invalid_field": "some value"}, n=5)

will raise:

ValidationError: 1 validation error for PetModel
my_pet.invalid_field
  Extra inputs are not permitted [type=extra_forbidden, input_value='some value', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/extra_forbidden

NB: As you can see, extra has the type ExtraValues now, and its value will get validated by ConfigDict. This means it's not possible to accidentally provide an unsupported value for extra (e.g. having a typo), i.e. something like ConfigDict(extra="fordib") will fail with a SchemaError.

Pydantic v1

You can use the extra field in the Config class to forbid extra attributes during model initialisation (by default, additional attributes will be ignored).

For example:

from pydantic import BaseModel, Extra

class Pet(BaseModel):
    name: str

    class Config:
        extra = Extra.forbid

data = {
    "name": "some name",
    "some_extra_field": "some value",
}

my_pet = Pet.parse_obj(data)   # <- effectively the same as Pet(**pet_data)

will raise a VaidationError:

ValidationError: 1 validation error for Pet
some_extra_field
  extra fields not permitted (type=value_error.extra)

Works as well when the model is "nested", e.g.:

class PetModel(BaseModel):
    my_pet: Pet
    n: int

pet_data = {
    "my_pet": {"name": "Some Name", "invalid_field": "some value"},
    "n": 5,
}

pet_model = PetModel.parse_obj(pet_data)
# Effectively the same as
# pet_model = PetModel(my_pet={"name": "Some Name", "invalid_field": "some value"}, n=5)

will raise:

ValidationError: 1 validation error for PetModel
my_pet -> invalid_field
  extra fields not permitted (type=value_error.extra)