Configuration

Behaviour of Pydantic can be controlled via the BaseModel.model_config, and as an argument to TypeAdapter.

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]
    """

1. 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]
    """

1. 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
   

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'}

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}