Configuration

The behaviour of Pydantic can be controlled via a variety of configuration values, documented on the ConfigDict class. This page describes how configuration can be specified for Pydantic's supported types.

Configuration on Pydantic models¶

On Pydantic models, configuration can be specified in two ways:

Using the model_config class attribute:

from pydantic import BaseModel, ConfigDict, ValidationError


class Model(BaseModel):
    model_config = ConfigDict(str_max_length=5)  # (1)!

    v: str


try:
    m = Model(v='abcdef')
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    v
      String should have at most 5 characters [type=string_too_long, input_value='abcdef', input_type=str]
    """

A plain dictionary (i.e. {'str_max_length': 5}) can also be used.

Note

In Pydantic V1, the Config class was used. This is still supported, but deprecated.

Using class arguments:
```
from pydantic import BaseModel


class Model(BaseModel, frozen=True):
    a: str  # (1)!
```
1. Unlike the model_config class attribute, static type checkers will recognize the frozen argument, and so any instance mutation will be flagged as an type checking error.

Configuration on Pydantic dataclasses¶

Pydantic dataclasses also support configuration (read more in the dedicated section).

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


@dataclass(config=ConfigDict(str_max_length=10, validate_assignment=True))
class User:
    name: str


user = User(name='John Doe')
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]
    """

Configuration on `TypeAdapter`¶

Type adapters (using the TypeAdapter class) support configuration, by providing a config argument.

from pydantic import ConfigDict, TypeAdapter

ta = TypeAdapter(list[str], config=ConfigDict(coerce_numbers_to_str=True))

print(ta.validate_python([1, 2]))
#> ['1', '2']

Configuration on other supported types¶

If you are using standard library dataclasses or TypedDict classes, the configuration can be set in two ways:

Using the __pydantic_config__ class attribute:

from dataclasses import dataclass

from pydantic import ConfigDict


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

    id: int
    name: str = 'John Doe'

Using the with_config decorator (this avoids static type checking errors with TypedDict):

from typing_extensions import TypedDict

from pydantic import ConfigDict, with_config


@with_config(ConfigDict(str_to_lower=True))
class Model(TypedDict):
    x: str

Change behaviour globally¶

If you wish to change the behaviour of Pydantic globally, you can create your own custom parent class with a custom configuration, as the configuration 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 provide configuration to the subclasses, it will be merged with the parent configuration:

from pydantic import BaseModel, ConfigDict


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


class Model(Parent):
    model_config = ConfigDict(str_to_lower=True)

    x: str


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

Warning

If your model inherits from multiple bases, Pydantic currently doesn't follow the MRO. For more details, see this issue.

Configuration propagation¶

Note that when using types that support configuration as field annotations on other types, configuration will not be propagated. In the following example, each model has its own "configuration boundary":

from pydantic import BaseModel, ConfigDict


class User(BaseModel):
    name: str


class Parent(BaseModel):
    user: User

    model_config = ConfigDict(str_max_length=2)


print(Parent(user={'name': 'John Doe'}))
#> user=User(name='John Doe')