Fields

API Documentation

pydantic.fields.Field

In this section, we will go through the available mechanisms to customize Pydantic model fields: default values, JSON Schema metadata, constraints, etc.

To do so, the Field() function is used a lot, and behaves the same way as the standard library field() function for dataclasses:

from pydantic import BaseModel, Field


class Model(BaseModel):
    name: str = Field(frozen=True)

Note

Even though name is assigned a value, it is still required and has no default value. If you want to emphasize on the fact that a value must be provided, you can use the ellipsis:

class Model(BaseModel):
    name: str = Field(..., frozen=True)

However, its usage is discouraged as it doesn’t play well with static type checkers.

The annotated pattern

To apply constraints or attach Field() functions to a model field, Pydantic supports the Annotated typing construct to attach metadata to an annotation:

from typing_extensions import Annotated

from pydantic import BaseModel, Field, WithJsonSchema


class Model(BaseModel):
    name: Annotated[str, Field(strict=True), WithJsonSchema({'extra': 'data'})]

As far as static type checkers are concerned, name is still typed as str, but Pydantic leverages the available metadata to add validation logic, type constraints, etc.

Using this pattern has some advantages:

• Using the f: <type> = Field(...) form can be confusing and might trick users into thinking f has a default value, while in reality it is still required.
• You can provide an arbitrary amount of metadata elements for a field. As shown in the example above, the Field() function only supports a limited set of constraints/metadata, and you may have to use different Pydantic utilities such as WithJsonSchema in some cases.
• Types can be made reusable (see the documentation on custom types using this pattern).

However, note that certain arguments to the Field() function (namely, default, default_factory, and alias) are taken into account by static type checkers to synthesize a correct __init__ method. The annotated pattern is not understood by them, so you should use the normal assignment form instead.

Tip

The annotated pattern can also be used to add metadata to specific parts of the type. For instance, validation constraints can be added this way:

from typing import List

from typing_extensions import Annotated

from pydantic import BaseModel, Field


class Model(BaseModel):
    int_list: List[Annotated[int, Field(gt=0)]]
    # Valid: [1, 3]
    # Invalid: [-1, 2]

Default values

The default parameter is used to define a default value for a field.

from pydantic import BaseModel, Field


class User(BaseModel):
    name: str = Field(default='John Doe')


user = User()
print(user)
#> name='John Doe'

Note

If you use the Optional annotation, it doesn’t mean that the field has a default value of None!

You can also use default_factory (but not both at the same time) to define a callable that will be called to generate a default value.

from uuid import uuid4

from pydantic import BaseModel, Field


class User(BaseModel):
    id: str = Field(default_factory=lambda: uuid4().hex)

The default factory can also take a single required argument, in which the case the already validated data will be passed as a dictionary.

from pydantic import BaseModel, EmailStr, Field


class User(BaseModel):
    email: EmailStr
    username: str = Field(default_factory=lambda data: data['email'])


user = User(email='[email protected]')
print(user.username)
#> [email protected]

The data argument will only contain the already validated data, based on the order of model fields (the above example would fail if username were to be defined before email).

Validate default values

By default, Pydantic will not validate default values. The validate_default field parameter (or the validate_default configuration value) can be used to enable this behavior:

from pydantic import BaseModel, Field, ValidationError


class User(BaseModel):
    age: int = Field(default='twelve', validate_default=True)


try:
    user = User()
except ValidationError as e:
    print(e)
    """
    1 validation error for User
    age
      Input should be a valid integer, unable to parse string as an integer [type=int_parsing, input_value='twelve', input_type=str]
    """

Mutable default values

A common source of bugs in Python is to use a mutable object as a default value for a function or method argument, as the same instance ends up being reused in each call.

The dataclasses module actually raises an error in this case, indicating that you should use a default factory instead.

While the same thing can be done in Pydantic, it is not required. In the event that the default value is not hashable, Pydantic will create a deep copy of the default value when creating each instance of the model:

from typing import Dict, List

from pydantic import BaseModel


class Model(BaseModel):
    item_counts: List[Dict[str, int]] = [{}]


m1 = Model()
m1.item_counts[0]['a'] = 1
print(m1.item_counts)
#> [{'a': 1}]

m2 = Model()
print(m2.item_counts)
#> [{}]

Field aliases

Tip

Read more about aliases in the dedicated section.

For validation and serialization, you can define an alias for a field.

There are three ways to define an alias:

• Field(alias='foo')
• Field(validation_alias='foo')
• Field(serialization_alias='foo')

The alias parameter is used for both validation and serialization. If you want to use different aliases for validation and serialization respectively, you can use thevalidation_alias and serialization_alias parameters, which will apply only in their respective use cases.

Here is an example of using the alias parameter:

from pydantic import BaseModel, Field


class User(BaseModel):
  name: str = Field(alias='username')


user = User(username='johndoe')  # (1)
print(user)
#> name='johndoe'
print(user.model_dump(by_alias=True))  # (2)
#> {'username': 'johndoe'}

The alias 'username' is used for instance creation and validation.

We are using model_dump to convert the model into a serializable format.

You can see more details about model_dump in the API reference.

Note that the by_alias keyword argument defaults to False, and must be specified explicitly to dump models using the field (serialization) aliases.

When by_alias=True, the alias 'username' is also used during serialization.

If you want to use an alias only for validation, you can use the validation_alias parameter:

from pydantic import BaseModel, Field


class User(BaseModel):
  name: str = Field(validation_alias='username')


user = User(username='johndoe')  # (1)
print(user)
#> name='johndoe'
print(user.model_dump(by_alias=True))  # (2)
#> {'name': 'johndoe'}

The validation alias 'username' is used during validation.

The field name 'name' is used during serialization.

If you only want to define an alias for serialization, you can use the serialization_alias parameter:

from pydantic import BaseModel, Field


class User(BaseModel):
  name: str = Field(serialization_alias='username')


user = User(name='johndoe')  # (1)
print(user)
#> name='johndoe'
print(user.model_dump(by_alias=True))  # (2)
#> {'username': 'johndoe'}

The field name 'name' is used for validation.

The serialization alias 'username' is used for serialization.

Alias precedence and priority

In case you use alias together with validation_alias or serialization_alias at the same time, the validation_alias will have priority over alias for validation, and serialization_alias will have priority over alias for serialization.

If you use an alias_generator in the Model Config, you can control the order of precedence for specified field vs generated aliases via the alias_priority setting. You can read more about alias precedence here.

VSCode and Pyright users

In VSCode, if you use the Pylance extension, you won’t see a warning when instantiating a model using a field’s alias:

from pydantic import BaseModel, Field


class User(BaseModel):
  name: str = Field(alias='username')


user = User(username='johndoe')  # (1)

VSCode will NOT show a warning here.

When the 'alias' keyword argument is specified, even if you set populate_by_name to True in the Model Config, VSCode will show a warning when instantiating a model using the field name (though it will work at runtime) — in this case, 'name':

from pydantic import BaseModel, ConfigDict, Field


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

  name: str = Field(alias='username')


user = User(name='johndoe')  # (1)

VSCode will show a warning here.

To “trick” VSCode into preferring the field name, you can use the str function to wrap the alias value. With this approach, though, a warning is shown when instantiating a model using the alias for the field:

from pydantic import BaseModel, ConfigDict, Field


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

  name: str = Field(alias=str('username'))  # noqa: UP018


user = User(name='johndoe')  # (1)
user = User(username='johndoe')  # (2)

Now VSCode will NOT show a warning

VSCode will show a warning here, though

This is discussed in more detail in this issue.

Validation Alias

Even though Pydantic treats alias and validation_alias the same when creating model instances, VSCode will not use the validation_alias in the class initializer signature. If you want VSCode to use the validation_alias in the class initializer, you can instead specify both an alias and serialization_alias, as the serialization_alias will override the alias during serialization:

from pydantic import BaseModel, Field


class MyModel(BaseModel):
    my_field: int = Field(validation_alias='myValidationAlias')

with:

from pydantic import BaseModel, Field


class MyModel(BaseModel):
    my_field: int = Field(
        ...,
        alias='myValidationAlias',
        serialization_alias='my_serialization_alias',
    )


m = MyModel(myValidationAlias=1)
print(m.model_dump(by_alias=True))
#> {'my_serialization_alias': 1}

Numeric Constraints

There are some keyword arguments that can be used to constrain numeric values:

• gt - greater than
• lt - less than
• ge - greater than or equal to
• le - less than or equal to
• multiple_of - a multiple of the given number
• allow_inf_nan - allow 'inf', '-inf', 'nan' values

Here’s an example:

from pydantic import BaseModel, Field


class Foo(BaseModel):
    positive: int = Field(gt=0)
    non_negative: int = Field(ge=0)
    negative: int = Field(lt=0)
    non_positive: int = Field(le=0)
    even: int = Field(multiple_of=2)
    love_for_pydantic: float = Field(allow_inf_nan=True)


foo = Foo(
    positive=1,
    non_negative=0,
    negative=-1,
    non_positive=0,
    even=2,
    love_for_pydantic=float('inf'),
)
print(foo)
"""
positive=1 non_negative=0 negative=-1 non_positive=0 even=2 love_for_pydantic=inf
"""

JSON Schema

In the generated JSON schema:

• gt and lt constraints will be translated to exclusiveMinimum and exclusiveMaximum.
• ge and le constraints will be translated to minimum and maximum.
• multiple_of constraint will be translated to multipleOf.

The above snippet will generate the following JSON Schema:

{
  "title": "Foo",
  "type": "object",
  "properties": {
    "positive": {
      "title": "Positive",
      "type": "integer",
      "exclusiveMinimum": 0
    },
    "non_negative": {
      "title": "Non Negative",
      "type": "integer",
      "minimum": 0
    },
    "negative": {
      "title": "Negative",
      "type": "integer",
      "exclusiveMaximum": 0
    },
    "non_positive": {
      "title": "Non Positive",
      "type": "integer",
      "maximum": 0
    },
    "even": {
      "title": "Even",
      "type": "integer",
      "multipleOf": 2
    },
    "love_for_pydantic": {
      "title": "Love For Pydantic",
      "type": "number"
    }
  },
  "required": [
    "positive",
    "non_negative",
    "negative",
    "non_positive",
    "even",
    "love_for_pydantic"
  ]
}

See the JSON Schema Draft 2020-12 for more details.

Constraints on compound types

In case you use field constraints with compound types, an error can happen in some cases. To avoid potential issues, you can use Annotated:

from typing import Optional

from typing_extensions import Annotated

from pydantic import BaseModel, Field


class Foo(BaseModel):
    positive: Optional[Annotated[int, Field(gt=0)]]
    # Can error in some cases, not recommended:
    non_negative: Optional[int] = Field(ge=0)

String Constraints

API Documentation

pydantic.types.StringConstraints

There are fields that can be used to constrain strings:

• min_length: Minimum length of the string.
• max_length: Maximum length of the string.
• pattern: A regular expression that the string must match.

Here’s an example:

from pydantic import BaseModel, Field


class Foo(BaseModel):
  short: str = Field(min_length=3)
  long: str = Field(max_length=10)
  regex: str = Field(pattern=r'^d*$')  # (1)


foo = Foo(short='foo', long='foobarbaz', regex='123')
print(foo)
#> short='foo' long='foobarbaz' regex='123'

Only digits are allowed.

JSON Schema

In the generated JSON schema:

• min_length constraint will be translated to minLength.
• max_length constraint will be translated to maxLength.
• pattern constraint will be translated to pattern.

The above snippet will generate the following JSON Schema:

{
  "title": "Foo",
  "type": "object",
  "properties": {
    "short": {
      "title": "Short",
      "type": "string",
      "minLength": 3
    },
    "long": {
      "title": "Long",
      "type": "string",
      "maxLength": 10
    },
    "regex": {
      "title": "Regex",
      "type": "string",
      "pattern": "^\\d*$"
    }
  },
  "required": [
    "short",
    "long",
    "regex"
  ]
}

Decimal Constraints

There are fields that can be used to constrain decimals:

• max_digits: Maximum number of digits within the Decimal. It does not include a zero before the decimal point or trailing decimal zeroes.
• decimal_places: Maximum number of decimal places allowed. It does not include trailing decimal zeroes.

Here’s an example:

from decimal import Decimal

from pydantic import BaseModel, Field


class Foo(BaseModel):
    precise: Decimal = Field(max_digits=5, decimal_places=2)


foo = Foo(precise=Decimal('123.45'))
print(foo)
#> precise=Decimal('123.45')

Dataclass Constraints

There are fields that can be used to constrain dataclasses:

• init: Whether the field should be included in the __init__ of the dataclass.
• init_var: Whether the field should be seen as an init-only field in the dataclass.
• kw_only: Whether the field should be a keyword-only argument in the constructor of the dataclass.

Here’s an example:

from pydantic import BaseModel, Field
from pydantic.dataclasses import dataclass


@dataclass
class Foo:
  bar: str
  baz: str = Field(init_var=True)
  qux: str = Field(kw_only=True)


class Model(BaseModel):
  foo: Foo


model = Model(foo=Foo('bar', baz='baz', qux='qux'))
print(model.model_dump())  # (1)
#> {'foo': {'bar': 'bar', 'qux': 'qux'}}

The baz field is not included in the model_dump() output, since it is an init-only field.

Field Representation

The parameter repr can be used to control whether the field should be included in the string representation of the model.

from pydantic import BaseModel, Field


class User(BaseModel):
  name: str = Field(repr=True)  # (1)
  age: int = Field(repr=False)


user = User(name='John', age=42)
print(user)
#> name='John'

This is the default value.

Discriminator

The parameter discriminator can be used to control the field that will be used to discriminate between different models in a union. It takes either the name of a field or a Discriminator instance. The Discriminator approach can be useful when the discriminator fields aren’t the same for all the models in the Union.

The following example shows how to use discriminator with a field name:

from typing import Literal, Union

from pydantic import BaseModel, Field


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


class Dog(BaseModel):
  pet_type: Literal['dog']
  age: int


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


print(Model.model_validate({'pet': {'pet_type': 'cat', 'age': 12}}))  # (1)
#> pet=Cat(pet_type='cat', age=12)

See more about [Validating data] in the [Models] page.

The following example shows how to use the discriminator keyword argument with a Discriminator instance:

from typing import Literal, Union

from typing_extensions import Annotated

from pydantic import BaseModel, Discriminator, Field, Tag


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


class Dog(BaseModel):
    pet_kind: Literal['dog']
    age: int


def pet_discriminator(v):
    if isinstance(v, dict):
        return v.get('pet_type', v.get('pet_kind'))
    return getattr(v, 'pet_type', getattr(v, 'pet_kind', None))


class Model(BaseModel):
    pet: Union[Annotated[Cat, Tag('cat')], Annotated[Dog, Tag('dog')]] = Field(
        discriminator=Discriminator(pet_discriminator)
    )


print(repr(Model.model_validate({'pet': {'pet_type': 'cat', 'age': 12}})))
#> Model(pet=Cat(pet_type='cat', age=12))

print(repr(Model.model_validate({'pet': {'pet_kind': 'dog', 'age': 12}})))
#> Model(pet=Dog(pet_kind='dog', age=12))

You can also take advantage of Annotated to define your discriminated unions. See the Discriminated Unions docs for more details.

Strict Mode

The strict parameter on a Field specifies whether the field should be validated in “strict mode”. In strict mode, Pydantic throws an error during validation instead of coercing data on the field where strict=True.

from pydantic import BaseModel, Field


class User(BaseModel):
  name: str = Field(strict=True)  # (1)
  age: int = Field(strict=False)


user = User(name='John', age='42')  # (2)
print(user)
#> name='John' age=42

This is the default value.

The age field is not validated in the strict mode. Therefore, it can be assigned a string.

See Strict Mode for more details.

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

Immutability

The parameter frozen is used to emulate the frozen dataclass behaviour. It is used to prevent the field from being assigned a new value after the model is created (immutability).

See the frozen dataclass documentation for more details.

from pydantic import BaseModel, Field, ValidationError


class User(BaseModel):
  name: str = Field(frozen=True)
  age: int


user = User(name='John', age=42)

try:
  user.name = 'Jane'  # (1)
except ValidationError as e:
  print(e)
  """
  1 validation error for User
  name
    Field is frozen [type=frozen_field, input_value='Jane', input_type=str]
  """

Since name field is frozen, the assignment is not allowed.

Exclude

The exclude parameter can be used to control which fields should be excluded from the model when exporting the model.

See the following example:

from pydantic import BaseModel, Field


class User(BaseModel):
  name: str
  age: int = Field(exclude=True)


user = User(name='John', age=42)
print(user.model_dump())  # (1)
#> {'name': 'John'}

The age field is not included in the model_dump() output, since it is excluded.

See the Serialization section for more details.

Deprecated fields

The deprecated parameter can be used to mark a field as being deprecated. Doing so will result in:

• a runtime deprecation warning emitted when accessing the field.
• "deprecated": true being set in the generated JSON schema.

You can set the deprecated parameter as one of:

• A string, which will be used as the deprecation message.
• An instance of the warnings.deprecated decorator (or the typing_extensions backport).
• A boolean, which will be used to mark the field as deprecated with a default 'deprecated' deprecation message.

deprecated as a string

from typing_extensions import Annotated

from pydantic import BaseModel, Field


class Model(BaseModel):
    deprecated_field: Annotated[int, Field(deprecated='This is deprecated')]


print(Model.model_json_schema()['properties']['deprecated_field'])
#> {'deprecated': True, 'title': 'Deprecated Field', 'type': 'integer'}

deprecated via the warnings.deprecated decorator

Note

You can only use the deprecated decorator in this way if you have typing_extensions >= 4.9.0 installed.

import importlib.metadata

from packaging.version import Version
from typing_extensions import Annotated, deprecated

from pydantic import BaseModel, Field

if Version(importlib.metadata.version('typing_extensions')) >= Version('4.9'):

    class Model(BaseModel):
        deprecated_field: Annotated[int, deprecated('This is deprecated')]

        # Or explicitly using `Field`:
        alt_form: Annotated[
            int, Field(deprecated=deprecated('This is deprecated'))
        ]

deprecated as a boolean

from typing_extensions import Annotated

from pydantic import BaseModel, Field


class Model(BaseModel):
    deprecated_field: Annotated[int, Field(deprecated=True)]


print(Model.model_json_schema()['properties']['deprecated_field'])
#> {'deprecated': True, 'title': 'Deprecated Field', 'type': 'integer'}

Support for category and stacklevel

The current implementation of this feature does not take into account the category and stacklevel arguments to the deprecated decorator. This might land in a future version of Pydantic.

Accessing a deprecated field in validators

When accessing a deprecated field inside a validator, the deprecation warning will be emitted. You can use catch_warnings to explicitly ignore it:

import warnings

from typing_extensions import Self

from pydantic import BaseModel, Field, model_validator


class Model(BaseModel):
    deprecated_field: int = Field(deprecated='This is deprecated')

    @model_validator(mode='after')
    def validate_model(self) -> Self:
        with warnings.catch_warnings():
            warnings.simplefilter('ignore', DeprecationWarning)
            self.deprecated_field = self.deprecated_field * 2

Customizing JSON Schema

Some field parameters are used exclusively to customize the generated JSON schema. The parameters in question are:

• title
• description
• examples
• json_schema_extra

Read more about JSON schema customization / modification with fields in the Customizing JSON Schema section of the JSON schema docs.

The computed_field decorator

API Documentation

computed_field

The computed_field decorator can be used to include property or cached_property attributes when serializing a model or dataclass. The property will also be taken into account in the JSON Schema (in serialization mode).

Note

Properties can be useful for fields that are computed from other fields, or for fields that are expensive to be computed (and thus, are cached if using cached_property).

However, note that Pydantic will not perform any additional logic on the wrapped property (validation, cache invalidation, etc.).

Here’s an example of the JSON schema (in serialization mode) generated for a model with a computed field:

from pydantic import BaseModel, computed_field


class Box(BaseModel):
    width: float
    height: float
    depth: float

    @computed_field
    @property  # (1)
    def volume(self) -> float:
        return self.width * self.height * self.depth


print(Box.model_json_schema(mode='serialization'))
"""
{
    'properties': {
        'width': {'title': 'Width', 'type': 'number'},
        'height': {'title': 'Height', 'type': 'number'},
        'depth': {'title': 'Depth', 'type': 'number'},
        'volume': {'readOnly': True, 'title': 'Volume', 'type': 'number'},
    },
    'required': ['width', 'height', 'depth', 'volume'],
    'title': 'Box',
    'type': 'object',
}
"""

Here’s an example using the model_dump method with a computed field:

from pydantic import BaseModel, computed_field


class Box(BaseModel):
  width: float
  height: float
  depth: float

  @computed_field
  @property  # (1)
  def volume(self) -> float:
      return self.width * self.height * self.depth


b = Box(width=1, height=2, depth=3)
print(b.model_dump())
#> {'width': 1.0, 'height': 2.0, 'depth': 3.0, 'volume': 6.0}

If not specified, [computed_field][pydantic.fields.computed_field] will implicitly convert the method to a [property][]. However, it is preferable to explicitly use the [@property][property] decorator for type checking purposes.

As with regular fields, computed fields can be marked as being deprecated:

from typing_extensions import deprecated

from pydantic import BaseModel, computed_field


class Box(BaseModel):
    width: float
    height: float
    depth: float

    @computed_field
    @property
    @deprecated("'volume' is deprecated")
    def volume(self) -> float:
        return self.width * self.height * self.depth