Model Config

Behaviour of pydantic can be controlled via the model_config attribute on a BaseModel.

Note

Before v2.0, the Config class was used. This is still supported, but deprecated.

from pydantic import BaseModel, ConfigDict, ValidationError


class Model(BaseModel):
    model_config = ConfigDict(str_max_length=10)

    v: str


try:
    m = Model(v='x' * 20)
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    v
      String should have at most 10 characters [type=string_too_long, input_value='xxxxxxxxxxxxxxxxxxxx', input_type=str]
    """

Also, you can specify config options as model class kwargs:

from pydantic import BaseModel, ValidationError


class Model(BaseModel, extra='forbid'):  # (1)!
    a: str


try:
    Model(a='spam', b='oh no')
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    b
      Extra inputs are not permitted [type=extra_forbidden, input_value='oh no', input_type=str]
    """

See the Extra Attributes section for more details.

Similarly, if using the @dataclass decorator from pydantic:

from datetime import datetime

from pydantic import ConfigDict, ValidationError
from pydantic.dataclasses import dataclass

config = ConfigDict(str_max_length=10, validate_assignment=True)


@dataclass(config=config)  # (1)!
class User:
    id: int
    name: str = 'John Doe'
    signup_ts: datetime = None


user = User(id='42', signup_ts='2032-06-21T12:00')
try:
    user.name = 'x' * 20
except ValidationError as e:
    print(e)
    """
    1 validation error for User
    name
      String should have at most 10 characters [type=string_too_long, input_value='xxxxxxxxxxxxxxxxxxxx', input_type=str]
    """

If using the dataclass from the standard library or TypedDict, you should use __pydantic_config__ instead. See:

from dataclasses import dataclass
from datetime import datetime

from pydantic import ConfigDict


@dataclass
class User:
    __pydantic_config__ = ConfigDict(strict=True)

    id: int
    name: str = 'John Doe'
    signup_ts: datetime = None

Options¶

See the ConfigDict API documentation for the full list of settings.

Change behaviour globally¶

If you wish to change the behaviour of Pydantic globally, you can create your own custom BaseModel with custom model_config since the config is inherited:

from pydantic import BaseModel, ConfigDict


class Parent(BaseModel):
    model_config = ConfigDict(extra='allow')


class Model(Parent):
    x: str


m = Model(x='foo', y='bar')
print(m.model_dump())
#> {'x': 'foo', 'y': 'bar'}

Since Parent is a subclass of BaseModel, it will inherit the model_config attribute. This means that Model will have extra='allow' by default.

If you add a model_config to the Model class, it will merge with the model_config from Parent:

from pydantic import BaseModel, ConfigDict


class Parent(BaseModel):
    model_config = ConfigDict(extra='allow')


class Model(Parent):
    model_config = ConfigDict(str_to_lower=True)  # (1)!

    x: str


m = Model(x='FOO', y='bar')
print(m.model_dump())
#> {'x': 'foo', 'y': 'bar'}
print(m.model_config)
#> {'extra': 'allow', 'str_to_lower': True}

Alias Generator¶

If data source field names do not match your code style (e. g. CamelCase fields), you can automatically generate aliases using alias_generator:

from pydantic import BaseModel, ConfigDict


def to_camel(string: str) -> str:
    return ''.join(word.capitalize() for word in string.split('_'))


class Voice(BaseModel):
    model_config = ConfigDict(alias_generator=to_camel)

    name: str
    language_code: str


voice = Voice(Name='Filiz', LanguageCode='tr-TR')
print(voice.language_code)
#> tr-TR
print(voice.model_dump(by_alias=True))
#> {'Name': 'Filiz', 'LanguageCode': 'tr-TR'}

Here camel case refers to "upper camel case" aka pascal case e.g. CamelCase. If you'd like instead to use lower camel case e.g. camelCase, instead use the to_lower_camel function.

Alias Precedence¶

If you specify an alias on the Field, it will take precedence over the generated alias by default:

from pydantic import BaseModel, ConfigDict, Field


def to_camel(string: str) -> str:
    return ''.join(word.capitalize() for word in string.split('_'))


class Voice(BaseModel):
    model_config = ConfigDict(alias_generator=to_camel)

    name: str
    language_code: str = Field(alias='lang')


voice = Voice(Name='Filiz', lang='tr-TR')
print(voice.language_code)
#> tr-TR
print(voice.model_dump(by_alias=True))
#> {'Name': 'Filiz', 'lang': 'tr-TR'}

Alias Priority¶

You may set alias_priority on a field to change this behavior:

alias_priority=2 the alias will not be overridden by the alias generator.
alias_priority=1 the alias will be overridden by the alias generator.
alias_priority not set, the alias will be overridden by the alias generator.

The same precedence applies to validation_alias and serialization_alias. See more about the different field aliases under field aliases.

Extra Attributes¶

You can configure how pydantic handles the attributes that are not defined in the model:

allow - Allow any extra attributes.
forbid - Forbid any extra attributes.
ignore - Ignore any extra attributes.

The default value is 'ignore'.

from pydantic import BaseModel, ConfigDict


class User(BaseModel):
    model_config = ConfigDict(extra='ignore')  # (1)!

    name: str


user = User(name='John Doe', age=20)  # (2)!
print(user)
#> name='John Doe'

This is the the default behaviour.
The age argument is ignored.

Instead, with extra='allow', the age argument is included:

from pydantic import BaseModel, ConfigDict


class User(BaseModel):
    model_config = ConfigDict(extra='allow')

    name: str


user = User(name='John Doe', age=20)  # (1)!
print(user)
#> name='John Doe' age=20

The age argument is included.

With extra='forbid', an error is raised:

from pydantic import BaseModel, ConfigDict, ValidationError


class User(BaseModel):
    model_config = ConfigDict(extra='forbid')

    name: str


try:
    User(name='John Doe', age=20)
except ValidationError as e:
    print(e)
    """
    1 validation error for User
    age
      Extra inputs are not permitted [type=extra_forbidden, input_value=20, input_type=int]
    """

Populate by Name¶

In case you set an alias, you can still populate the model by the original name.

You need to set populate_by_name=True in the model_config:

from pydantic import BaseModel, ConfigDict, Field


class User(BaseModel):
    model_config = ConfigDict(populate_by_name=True)

    name: str = Field(alias='full_name')  # (1)!
    age: int


user = User(full_name='John Doe', age=20)  # (2)!
print(user)
#> name='John Doe' age=20
user = User(name='John Doe', age=20)  # (3)!
print(user)
#> name='John Doe' age=20

The field 'name' has an alias 'full_name'.
The model is populated by the alias 'full_name'.
The model is populated by the field name 'name'.

Validate Assignment¶

The default behavior of Pydantic is to validate the data when the model is created.

In case the user changes the data after the model is created, the model is not revalidated.

from pydantic import BaseModel


class User(BaseModel):
    name: str


user = User(name='John Doe')  # (1)!
print(user)
#> name='John Doe'
user.name = 123  # (1)!
print(user)
#> name=123

The validation happens only when the model is created.
The validation does not happen when the data is changed.

In case you want to revalidate the model when the data is changed, you can use validate_assignment=True:

from pydantic import BaseModel, ValidationError


class User(BaseModel, validate_assignment=True):  # (1)!
    name: str


user = User(name='John Doe')  # (2)!
print(user)
#> name='John Doe'
try:
    user.name = 123  # (3)!
except ValidationError as e:
    print(e)
    """
    1 validation error for User
    name
      Input should be a valid string [type=string_type, input_value=123, input_type=int]
    """

You can either use class keyword arguments, or model_config to set validate_assignment=True.
The validation happens when the model is created.
The validation also happens when the data is changed.

Revalidate instances¶

By default, model and dataclass instances are not revalidated during validation.

from typing import List

from pydantic import BaseModel


class User(BaseModel, revalidate_instances='never'):  # (1)!
    hobbies: List[str]


class SubUser(User):
    sins: List[str]


class Transaction(BaseModel):
    user: User


my_user = User(hobbies=['reading'])
t = Transaction(user=my_user)
print(t)
#> user=User(hobbies=['reading'])

my_user.hobbies = [1]  # (2)!
t = Transaction(user=my_user)  # (3)!
print(t)
#> user=User(hobbies=[1])

my_sub_user = SubUser(hobbies=['scuba diving'], sins=['lying'])
t = Transaction(user=my_sub_user)
print(t)
#> user=SubUser(hobbies=['scuba diving'], sins=['lying'])

revalidate_instances is set to 'never' by **default.
The assignment is not validated, unless you set validate_assignment to True in the model's config.
Since revalidate_instances is set to never, this is not revalidated.

If you want to revalidate instances during validation, you can set revalidate_instances to 'always' in the model's config.

from typing import List

from pydantic import BaseModel, ValidationError


class User(BaseModel, revalidate_instances='always'):  # (1)!
    hobbies: List[str]


class SubUser(User):
    sins: List[str]


class Transaction(BaseModel):
    user: User


my_user = User(hobbies=['reading'])
t = Transaction(user=my_user)
print(t)
#> user=User(hobbies=['reading'])

my_user.hobbies = [1]
try:
    t = Transaction(user=my_user)  # (2)!
except ValidationError as e:
    print(e)
    """
    1 validation error for Transaction
    user.hobbies.0
      Input should be a valid string [type=string_type, input_value=1, input_type=int]
    """

my_sub_user = SubUser(hobbies=['scuba diving'], sins=['lying'])
t = Transaction(user=my_sub_user)
print(t)  # (3)!
#> user=User(hobbies=['scuba diving'])

revalidate_instances is set to 'always'.
The model is revalidated, since revalidate_instances is set to 'always'.
Using 'never' we would have gotten user=SubUser(hobbies=['scuba diving'], sins=['lying']).

It's also possible to set revalidate_instances to 'subclass-instances' to only revalidate instances of subclasses of the model.

from typing import List

from pydantic import BaseModel


class User(BaseModel, revalidate_instances='subclass-instances'):  # (1)!
    hobbies: List[str]


class SubUser(User):
    sins: List[str]


class Transaction(BaseModel):
    user: User


my_user = User(hobbies=['reading'])
t = Transaction(user=my_user)
print(t)
#> user=User(hobbies=['reading'])

my_user.hobbies = [1]
t = Transaction(user=my_user)  # (2)!
print(t)
#> user=User(hobbies=[1])

my_sub_user = SubUser(hobbies=['scuba diving'], sins=['lying'])
t = Transaction(user=my_sub_user)
print(t)  # (3)!
#> user=User(hobbies=['scuba diving'])

revalidate_instances is set to 'subclass-instances'.
This is not revalidated, since my_user is not a subclass of User.
Using 'never' we would have gotten user=SubUser(hobbies=['scuba diving'], sins=['lying']).

Strict Mode¶

By default, Pydantic attempts to coerce values to the correct type, when possible.

There are situations in which you may want to disable this behavior, and instead raise an error if a value's type does not match the field's type annotation.

To configure strict mode for all fields on a model, you can set model_config = ConfigDict(strict=True) on the model.

from pydantic import BaseModel, ConfigDict


class Model(BaseModel):
    model_config = ConfigDict(strict=True)

    name: str
    age: int

See Strict Mode for more details.

See the Conversion Table for more details on how Pydantic converts data in both strict and lax modes.

Arbitrary Types Allowed¶

You can allow arbitrary types using the arbitrary_types_allowed setting in the model's config:

from pydantic import BaseModel, ConfigDict, ValidationError


# This is not a pydantic model, it's an arbitrary class
class Pet:
    def __init__(self, name: str):
        self.name = name


class Model(BaseModel):
    model_config = ConfigDict(arbitrary_types_allowed=True)

    pet: Pet
    owner: str


pet = Pet(name='Hedwig')
# A simple check of instance type is used to validate the data
model = Model(owner='Harry', pet=pet)
print(model)
#> pet=<__main__.Pet object at 0x0123456789ab> owner='Harry'
print(model.pet)
#> <__main__.Pet object at 0x0123456789ab>
print(model.pet.name)
#> Hedwig
print(type(model.pet))
#> <class '__main__.Pet'>
try:
    # If the value is not an instance of the type, it's invalid
    Model(owner='Harry', pet='Hedwig')
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    pet
      Input should be an instance of Pet [type=is_instance_of, input_value='Hedwig', input_type=str]
    """
# Nothing in the instance of the arbitrary type is checked
# Here name probably should have been a str, but it's not validated
pet2 = Pet(name=42)
model2 = Model(owner='Harry', pet=pet2)
print(model2)
#> pet=<__main__.Pet object at 0x0123456789ab> owner='Harry'
print(model2.pet)
#> <__main__.Pet object at 0x0123456789ab>
print(model2.pet.name)
#> 42
print(type(model2.pet))
#> <class '__main__.Pet'>

Protected Namespaces¶

Pydantic prevents collisions between model attributes and BaseModel's own methods by namespacing them with the prefix model_.

import warnings

from pydantic import BaseModel

warnings.filterwarnings('error')  # Raise warnings as errors

try:

    class Model(BaseModel):
        model_prefixed_field: str

except UserWarning as e:
    print(e)
    """
    Field "model_prefixed_field" has conflict with protected namespace "model_".

    You may be able to resolve this warning by setting `model_config['protected_namespaces'] = ()`.
    """

You can customize this behavior using the protected_namespaces setting:

import warnings

from pydantic import BaseModel, ConfigDict

warnings.filterwarnings('error')  # Raise warnings as errors

try:

    class Model(BaseModel):
        model_prefixed_field: str
        also_protect_field: str

        model_config = ConfigDict(
            protected_namespaces=('protect_me_', 'also_protect_')
        )

except UserWarning as e:
    print(e)
    """
    Field "also_protect_field" has conflict with protected namespace "also_protect_".

    You may be able to resolve this warning by setting `model_config['protected_namespaces'] = ('protect_me_',)`.
    """

While Pydantic will only emit a warning when an item is in a protected namespace but does not actually have a collision, an error is raised if there is an actual collision with an existing attribute:

from pydantic import BaseModel

try:

    class Model(BaseModel):
        model_validate: str

except NameError as e:
    print(e)
    """
    Field "model_validate" conflicts with member <bound method BaseModel.model_validate of <class 'pydantic.main.BaseModel'>> of protected namespace "model_".
    """

Hide Input in Errors¶

Pydantic shows the input value and type when it raises ValidationError during the validation.

from pydantic import BaseModel, ValidationError


class Model(BaseModel):
    a: str


try:
    Model(a=123)
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    a
      Input should be a valid string [type=string_type, input_value=123, input_type=int]
    """

You can hide the input value and type by setting the hide_input_in_errors config to True.

from pydantic import BaseModel, ConfigDict, ValidationError


class Model(BaseModel):
    a: str

    model_config = ConfigDict(hide_input_in_errors=True)


try:
    Model(a=123)
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    a
      Input should be a valid string [type=string_type]
    """