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Fields

API Documentation

pydantic.fields.Field

The Field function is used to customize and add metadata to fields of models.

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'

You can also use default_factory 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: int = Field(default_factory=lambda: uuid4().hex)

Info

The default and default_factory parameters are mutually exclusive.

Note

If you use typing.Optional, it doesn't mean that the field has a default value of None!

Field aliases

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 some example usage of 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'}
  1. The alias 'username' is used for instance creation and validation.

  2. 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'}
  1. The validation alias 'username' is used during validation.
  2. 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'}
  1. The field name 'name' is used for validation.
  2. 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.

You may also set alias_priority on a field to change this behavior.

You can read more about Alias Precedence in the Model Config documentation.

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)!
  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)!
  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:

from pydantic import BaseModel, ConfigDict, Field


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

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

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}

All of the above will likely also apply to other tools that respect the @typing.dataclass_transform decorator, such as Pyright.

AliasPath and AliasChoices

API Documentation

pydantic.fields.AliasPath
pydantic.fields.AliasChoices

Pydantic provides two special types for convenience when using validation_alias: AliasPath and AliasChoices.

The AliasPath is used to specify a path to a field using aliases. For example:

from pydantic import BaseModel, Field, AliasPath


class User(BaseModel):
    first_name: str = Field(validation_alias=AliasPath('names', 0))
    last_name: str = Field(validation_alias=AliasPath('names', 1))

user = User.model_validate({'names': ['John', 'Doe']})  # (1)!
print(user)
#> first_name='John' last_name='Doe'

  1. We are using model_validate to validate a dictionary using the field aliases.

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

In the 'first_name' field, we are using the alias 'names' and the index 0 to specify the path to the first name. In the 'last_name' field, we are using the alias 'names' and the index 1 to specify the path to the last name.

AliasChoices is used to specify a choice of aliases. For example:

from pydantic import BaseModel, Field, AliasChoices


class User(BaseModel):
    first_name: str = Field(validation_alias=AliasChoices('first_name', 'fname'))
    last_name: str = Field(validation_alias=AliasChoices('last_name', 'lname'))

user = User.model_validate({'fname': 'John', 'lname': 'Doe'})  # (1)!
print(user)
#> first_name='John' last_name='Doe'
user = User.model_validate({'first_name': 'John', 'lname': 'Doe'})  # (2)!
print(user)
#> first_name='John' last_name='Doe'
  1. We are using the second alias choice for both fields.
  2. We are using the first alias choice for the field 'first_name' and the second alias choice for the field 'last_name'.

You can also use AliasChoices with AliasPath:

from pydantic import BaseModel, Field, AliasPath, AliasChoices


class User(BaseModel):
    first_name: str = Field(validation_alias=AliasChoices('first_name', AliasPath('names', 0)))
    last_name: str = Field(validation_alias=AliasChoices('last_name', AliasPath('names', 1)))


user = User.model_validate({'first_name': 'John', 'last_name': 'Doe'})
print(user)
#> first_name='John' last_name='Doe'
user = User.model_validate({'names': ['John', 'Doe']})
print(user)
#> first_name='John' last_name='Doe'
user = User.model_validate({'names': ['John'], 'last_name': 'Doe'})
print(user)
#> first_name='John' last_name='Doe'

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.

String Constraints

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

Validate Default Values

The parameter validate_default can be used to control whether the default value of the field should be validated.

By default, the default value of the field is not validated.

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]
    """

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'
  1. 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.

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)
  1. See more about Helper Functions in the Models page.
from typing import Literal

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: Cat | Dog = Field(discriminator='pet_type')


print(Model.model_validate({'pet': {'pet_type': 'cat', 'age': 12}}))  # (1)!
#> pet=Cat(pet_type='cat', age=12)
  1. See more about Helper Functions in the Models page.

See the Discriminated Unions 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
  1. This is the default value.
  2. 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]
    """
  1. 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'}
  1. The age field is not included in the model_dump() output, since it is excluded.

See the Serialization section for more details.

Customizing JSON Schema

There are fields that exclusively to customise the generated JSON Schema:

  • title: The title of the field.
  • description: The description of the field.
  • examples: The examples of the field.
  • json_schema_extra: Extra JSON Schema properties to be added to the field.

Here's an example:

from pydantic import BaseModel, EmailStr, Field, SecretStr


class User(BaseModel):
    age: int = Field(description='Age of the user')
    email: EmailStr = Field(examples=['[email protected]'])
    name: str = Field(title='Username')
    password: SecretStr = Field(
        json_schema_extra={
            'title': 'Password',
            'description': 'Password of the user',
            'examples': ['123456'],
        }
    )


print(User.model_json_schema())
"""
{
    'properties': {
        'age': {
            'description': 'Age of the user',
            'title': 'Age',
            'type': 'integer',
        },
        'email': {
            'examples': ['[email protected]'],
            'format': 'email',
            'title': 'Email',
            'type': 'string',
        },
        'name': {'title': 'Username', 'type': 'string'},
        'password': {
            'description': 'Password of the user',
            'examples': ['123456'],
            'format': 'password',
            'title': 'Password',
            'type': 'string',
            'writeOnly': True,
        },
    },
    'required': ['age', 'email', 'name', 'password'],
    'title': 'User',
    'type': 'object',
}
"""